For anyone interested in monitoring how much heat it actually takes to stop stuff here's some math:

We'll use as our point of comparison, a coke can full of water (250 mL).
Since you have been holding it in your hand for a long time, the water is at body temperature (37 C)

The specific heat of water is 4.186 J/g per C, so it would take around 1046.5 Joules to heat up your can of water 1 C.

So how much energy is ~1000 Joules you ask?

A 9mm pistol bullet is around 500J, a big pistol bullet (~.44 magnum) is around 1000 joules, a rifle bullet (7.62 Soviet) is around 2000 joules.

So if our heat based shield is based off taking the energy out of the bullets (so they come to a complete stop in mid-air) and just fall down) and transferring that energy into our coke can.

So assuming our heat shield is 100% efficient, and some bad buy empties all 30 rounds of his AK into our heat shield, your coke can full of water goes to ~97 C, not even boiling yet.  But certainly hot enough to burn you if you holding it.  (Somewhere around 45-50 C is hot enough to scald your skin).  A magazine full of fire from a pistol (15 rounds of 9mm) is only enough to raise the temp of our coke can is barely enough heat to raise it from body temperature to hot enough to scald you.

So what do we learn from this?  Enchanted cans of Coke that provide you a fire based defensive shield that stops bullets can totally be tapped from cool and refreshing drink afterwards, as long as you don't get shot too much.   

Long post is long.

From a fluff to rules perspective, I'd prefer answer of "why does Harry do X?" to not be "because Harry is basically an idiot," when we totally don't get in text evidence to back that up.  For example, you can make plenty of 'anti-lore' claims against Harry, because he is quite wrong about stuff in the text.  But we don't really have any in text claims about how Harry is bad at whipping ass in combat evocation.  So, we want the rules for combat evocation to not totally shaft Harry for using one element for attack, and a different element for defense (and get nothing else out of splitting up his elements like that).

Depending on how you are allowed to move points around, you are totally rewarded for specializing in one element up to 5 refinement with the restrictive interpretation about how to allocate points, and up to 10 refinement with the loose one.  And that's only with putting points into specializations.  If we want to go with refinement for more powerful focus items, we can totally put more points into kicking ass with just one element.

How's that work?
There are two ways we can interpret how you are able to have specializations. 

The loose way: as long as your point total is ok, and your pyramid is ok, you are good.  So under the loose interpretation (Spirit Power +6, Spirit Control +5, Fire power +4, Fire Control +3, Water Power +2, Water Control +1) is totally ok.

The strict way: you are limited to increasing your specializations one point at the time, and you need to be able to show a valid path from your current specialization allocation to your stating point.  So (+6, ..., +1) is not OK, because you can't get from that total -2 points to another valid specialization allocation.

So, under the loose way, we can pay 10 refinement for a +6 and +5 in the elements we care about, and not even have to buy the other evocation elements.  And since evocations elements are all interchangeable anyway, why would we want to waste points on that?

Under the strict way, we have to go something like this:
Start
Spirit power +1
Refinement 1
Spirit Power +2, Spirit Control +1
Refinement 2
Spirit Power +2, Spirit Control +1, Fire Power +1, Fire Control +1
Refinement 3
Spirit Control +3, Spirit Power +2, Fire Power +1, Fire Control +1
Refinement 4
Spirit Control +3, Spirit Power +2, Water Power +2, Fire Power +1, Fire Control +1
Refinement 5
Spirit Power +4, Spirit Control +3, Water Power +2, Fire Power +1, Fire Control +1

And I think that's as far as you can go without buying other elements.  Someone who is better at playing Tower of Hanoi may come with with a path to +5,+4, etc. without going over 6 total places to put bonuses, but I don't think there is one.

There is probably a way to get more power at the same refresh level by stacking focus items and evocation specializations, but I'm not too interested in finding it right now.

But 5 refinement devoted solely to Evocation is a pretty big deal.  If we got with the loose interpretation, 10 refinement spent on evocation probably gets you into range to throw down with members of the senior council.

We'll take a look using the strict interpretation of specialization advancement, up to five refinement.

We'll take our evocation specialization wizard Conviction 3, Discipline 4
Evo specialization of Spirit Power +1
Focus items for +1 spirit (offence & defense; power & control) (4 slots)

What does he look like at different refinement levels (and assuming he isn't increasing his skills)?

“Other stuff” refers to his most powerful element combination without foci.

Start (Power, Control)
Offense (5,5) Defense (5,5), Other Stuff (4,4)
Refinement 1
Offense (6,6) Defense (6,6), Other Stuff (5,5)
Refinement 2
Offense (6,6) Defense (6,6),  Other Stuff (5,5), plus more skill in elements we don't care about
Refinement 3
Offense (6,8) Defense (6,8),  Other Stuff (5,7), we can switch how our focus items are allocated to get to (7,7)
Refinement 4
Offense (6,8) Defense (6,8), Other Stuff (5,7), plus more skill in elements we don't care about
Refinement 5
Offense (8,8) Defense (8,8), Other Stuff (7,7)

Now, what about the guy who tries to attack and defend with two different elements?

We'll take our evocation specialization wizard Conviction 3, Discipline 4
We assume he'll attack with fire and defend with water.  He wants to be even in attack and defense, but will favor defense if  he has to choose.

Evo specialization of Water Power +1
Focus items from +1 (offence & defense; power & control) (4 slots)
Start
Water Power +1
Offense (4,5) Defense (5,5), Other stuff (4,4)

Refinement 1
Water Power +2, Water Control +1
Offense (4,5) Defense (6,6), Other Stuff (5,5)

Refinement 2
Water Power +2, Water Control +1, Fire Power 1, Fire Control 1
Offense (5,6) Defense (6,6), Other Stuff (5,5)

Refinement 3
Water Power +2, Water Control +1, Fire Power 2, Fire Control 1, Earth Power 1
Offense (6,6) Defense (6,6), Other Stuff (5,5)

Refinement 4
Water Power +3, Water Control +2, Fire Power 2, Fire Control 1, Earth Power1
Offense (6,6) Defense (7,7), Other Stuff (6,6)

Refinement 5
Water Power +3, Water Control +2, Fire Power 4, Fire Control 1, Earth Power1
Offense (8,6) Defense (7,7), Other Stuff (6,6 or 7,5), can shift offensive focus item for (7,7)

-------

So how do they compare?
Start:
Specialist
Offense (5,5) Defense (5,5), Other Stuff (4,4)
2 Element Guy
Offense (4,5) Defense (5,5), Other stuff (4,4)

After 5 Refinement
Specialist:
Offense (8,8) Defense (8,8), Other Stuff (7,7)
2 Element Guy
Offense (7,7) Defense (7,7), Other Stuff (6,6 or 7,5)

So, at the start, 2 element guy really isn’t that far behind specialist guy.  But after 5 refinement, 2 element guy basically has to invoke an aspect every turn to keep up with specialist guy.  Does 2 element guy at least get some advantage of being able to do (7,5) Fire and (6,6) Water evocations compared to spirit guy being able to do (7,7) spirit stuff?  Not when all elements are equal, no.

With my proposed changes, 2 element guy can at least match spirit guy in area of effect with fire, and he has counterspells at (7,5) to compare to spirit guy’s veils (at 7,7).

---

Note that the same dynamic doesn’t apply to thaumaturgy specializations because they do different things.  You can’t substitute a divination of a bit of biomancy.

But even if the evocation elements did totally different things, there is still the incentive to specialize because your best evocation element determines both your attack and defense.

---

Another idea that I don’t have time to think about right now, is to make the advantages of different elements even greater, but add something like a negative aspect to each one that the GM can compel.

For example:
Fire, great for blasting, but hard to control
Fire can get a free area upgrade if you want one.
The negative aspect of fire is that tends to spread.  When you cast a fire spell, the GM can use a compel against you to cause the spell to effect one additional area.

Example, you are in combat, and call up a fire shield.  The GM uses the compel against you to make your fire shield protect some of the enemies as well.

Earth: Lasts a long time, but slow
Earth spells get a free duration increase. 
If you spend shifts for additional increase, you can another one free (so a Power 5 earth spell would be Power 5, duration 1; and could be made power 4, duration 3, or power 3, duration 4)
The negative aspect of earth is that it is very slow to get going.  When you cast an earth spell, the GM can use a compel against you to make you go last in the exchange.

Example, you want to use earth to call up a block to make a zone border.  The GM uses the compel against you to make you go last in this exchange, so maybe some of the baddies with have the chance to cross the zone before you can get the block up.

Something like this:

Air explicitly gets the ability to do the following:
Eavesdropping (investigation sub-ability)
Examination (investigation related ability), for example, you could walk into the bookstore and use an air effect to speed up your finding of Die Lied der Erlking.

The other elements can do detections to find their own element, for example, you could use earth to find the weapon points in a steel bridge, or something.  But air based detection lets you find almost anything.

Fire:
bonus area:
So if we have a power 5 spell that we want to effect a whole are, we'd normally pay 2 power for that, meaning that if would be power 3, area 1.
If it were a fire spell, we only have to pay 1 power for the first area (but 2 for all the following areas) so it would be power 4, area 1.

Earth:
same trick for duration.  If I want to make, say, an earthquake spell: I start with 8 power, I want it to effect 2 areas, and last for 1 extra exchange.  So that would normally be power 3, area 2, duration 1 spell.  But with the duration bonus for earth, it would be power 3, area 2, duration 2. 
In addition, when you make the roll to extend an earth magic spell, you get 1 free shift to duration if you succeed.  Example, I call up my basic earth shield, power 4.  But the next round, I want to to stick around, so i spend an action on increasing its duration.  I get four shifts for duration, so normally, my spell would stick around for another 4 exchanges.  But since it's an earth spell, it will stick around for additional 5 exchanges instead.

Water:
The counterspell ability is now based on your skill and power with water magic.  So any foci, specializations, etc that apply to water also apply to your counterspell ability.

Spirit:
veils, (which generally act as a stealth roll) and illusion maneuvers go here.

Hmm, after reading the rules more closely, I guess I have to agree with your interpretation.  You can throw around whatever power you can control with a rote, and just pay any extra stress that this may cause.

Assuming that every step up in weapon category is supposed to represent a weapon that is around 2-4 times more powerful than the previous weapon step.

So you go from
pistols Weapon: 2
rifles: weapon: 3
50 caliber sniper rifles (and similar) weapon: 4
~25mm cannon: weapon: 5
and tank shells are something like weapon: 8 or weapon: 9

So moving up from weapon 3 to weapon 4 actually is a bigger step than moving from weapon 2 to weapon 3.

But if counterspells aren't affiliated with any element, you can't get any better at them with refinement...  And I'd assume that they are supposed to be something you can get better at doing.

Hmm, how about this interpretation:

When you are the target of a magical maneuver, you generally have 2 defensive options: can can attempt to avoid the maneuver itself (just like you would any other attack) or you may attempt to resist the maneuver directly, which may allow you to use a different skill.

If you fail at whichever defensive option you choose, you are effected by the maneuver.  The placed aspect will generally stay for the duration of a scene, unless you make a maneuver yourself (generally, difficulty 3) to remove it.

Additional duration: when a maneuver spell with additional duration effects you, are are unable to attempt to remove the forced aspect until the natural duration of the spell expires.

Example:
Wizard bob throws the spell "Blinding Light, power 4, duration one, resisted with alertness" at a ghoul. 
Bob rolls a discipline result of 5, enough to control the spell. 
The ghoul has the choice of resisting the maneuver with either its alertness (Good) vs the power of the maneuver (4) or it's athletics (Superb, due to speed) vs the to hit roll of the maneuver (5).  It chooses to resist with Athletics, since the odds are better. 
The ghoul fails the roll, and gets the aspect blinded. 
Normally, the ghoul could attempt to remove this aspect with an alertness roll in the next exchange, but the added duration of the maneuver means it is unable to make the attempt until the exchange after that, when the spell duration will have ended.

[edit]
This still isn't too great, since you don't have to pay stress for regular maneuvers...
For example, I could use fists (or something) to inflict sand in the eyes.  You only have one skill option when defending against it, and I don't have to pay any stress to do that.

So an alternative is you resist the power of the maneuver directly with the appropriate skill, and the duration rule (above) is the advantage you get in exchange for paying stress to get the spell.

So the example would be:
Wizard Bob throws the spell "Blinding Light, power 4, duration one, resisted with alertness" at a ghoul. 
Bob rolls a discipline result of 5, enough to control the spell. 
The ghoul resists the power maneuver (4) with its alertness (Good)
The ghoul fails the roll, and gets the aspect blinded. 
Normally, the ghoul could attempt to remove this aspect with an alertness roll in the next exchange, but the added duration of the maneuver means it is unable to make the attempt until the exchange after that, when the spell duration will have ended.

So the advantages of spell based maneuvers are:
1 You generally get to choose the resistance skill, which means you can try to hit your opponent in a weak spot.
2 If you have enough power, you can make sure the aspect you stick them with will stick around for a while.

And skill based maneuvers, you don't have to pay stress for.

The disadvantage of this solution is that there is no allowance for magical blocks, which you would expect to block magical maneuvers...

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