Oh, absolutely, the +4 roll is statistically unlikely - I just tried to see if reaching a total of 20 shifts from Attack and Weapon Rating was plausible without the expenditure of half a ton (depending on Fate point token weight) or so of Fate Points... Which, it seems, it is.
When you then start adding Aspect tags that do cost Fate points into the equation, Mortals are likely to have more Fate points to consistently pour into their rolls than supernaturals...
Granted, but we're talking about maybe getting one such attack off extra really, unless they do a lot of prep (but prepping is nothing special, anyone can do it). Let's say a mortal has 5 spare refresh compared to a supernatural's 1. That's +8 on one attack per session more than what the supernatural gets. Let's say we add 5 more refresh after a lot of time and the mortal just keeps it as fate points. That's two such big attacks (which are unlikely to be 20 stress each). A Supernatural by this point could grab +5 on all his magic attacks (wizard with a 5 focus, and he has 2 refresh left over), go from inhuman to mythic strength (+4 stress on all physical attacks), etc. The supernaturals also get really nice defensive upgrades potentially.
Now, what a mortal does bring to the table is flexibility, to an extent. That can invoke for effect very easily, which I think is probably one of the most powerful uses of fate points (+2 sounds nice, but invoke for effects can be crazy-good). They can also use those fate points in social encounters, where it is harder to use magic or the like. Still, I think in the long run, mortals lose out.
Kind of like in the books, Murphy manages to keep up with Dresden for quite a while, but it really looks like she'll have to start using the sword to work in the big leagues.
Extra fate points are nice, but I think they generally are weaker than 1 refresh at higher levels -- and to be fair, a wizard could decide to stack up on fate points with extra refresh if he wanted, but it just isn't worth it.
I am not sure I'd say this is the system breaking down, per se, but it seems to me that pure mortals lose their luster at some point. I don't think it is in any of the starting power levels though. I'd
guess by Refresh 20 it would have happened, but that's hard to say without playtesting a lot. Wizards are more problematic though, given how foci and specializations work they can get real crazy compared to others.