By your definition, then, perfect clarity cannot exist.  So what's the point in discussing it?

It would make more sense to define clarity in terms of being able to communicate concepts to those reasonably equipped to understand the concept, but I doubt that the world's most skillful communicator could convey to, say, an infant how to fold a towel in a way that the infant could understand.  Which makes "clarity" a worthless concept.

While the book is far from precise on this subject, it's probably worth noting the following:

1) Inhuman Speed is described on YS178 as follows, my emphasis: "You are very fast, just past the edge of human capability."
2) I am led to believe based on a wiki entry that "The fastest human footspeed on record is 44.72 km/h (27.79 mph), seen during a 100-meter sprint (average speed between the 60th and the 80th meter) by Usain Bolt."

My parsing of this is that statement #1 is comparing an otherwise typical/average character that has acquired Inhuman Speed to the absolute best possible mundane human capability (as described by statement #2).

So, if I was to put numbers on paper, I'd probably say that a character with Average Athletics (+1) and Inhuman Speed can expect to reach about 30mph -- assuming a short sprint, average success, and no aspects invoked.  A more athletic character with Inhuman Speed and with more of an investment in moving as fast as possible (running a race, running from something that wants to eat him) would be noticably faster.

Based on this, I would say that a character with Inhuman Speed might be able to keep up with that 40mph train if he had a significant investment in speed-related stunts, athletics, etc, and rolled well.  Maybe.  A character with Superhuman Speed ought to be able to catch up to the train fairly easily.

The RAW for Mythic Speed indicates that it allows a character with no other prerequisites to "simply declare that you have no problem keeping up with a moving vehicle".  Another vague statement; this could be rules-lawyered to mean that a character with Mythic Speed could casually keep up with, say, a spacecraft -- which is technically a moving vehicle.  For the sake of sanity, I'm going to assume that they meant something much more mundane -- say, a car moving at freeway speeds (70mph).  And again, I'm going to assume that practically any character with Mythic Speed can achieve this, regardless of other abilities.

So here's a chart to play with, completely abitrarily recorded by me based on the above:

average human (sustained/sprint): 6mph/10mph
optimal human (sustained/sprint): 19mph/28mph
Inhuman Speed* (sustained/sprint): 20mph/30mph
Supernatural Speed* (sustained/sprint): 40mph/60mph
Mythic Speed* (sustained/sprint): 70mph/100mph?

(*) baseline for otherwise average human capabilities

Feel free to disagree!

Or it could be that the writer is fond of ironic understatement.

For example, if I said "Being shot in the head is generally a bad thing", you could argue that "generally" doesn't mean the same thing as "always", and that in fact my statement implies that quite often being shot in the head might well be a good thing (or at least a non-bad thing).  Logically, you would be correct.  But I still advise against shooting yourself in the head.

Similarly, the writer might have figured that the fact that it was very specifically described as a demonic co-pilot would be enough to convey to the reasonable reader that there was little expectation (more understatement, there!) of a warm and fuzzy, rainbow-colored, Lawful Good demonic co-pilot that really just wants to help you in any way that would make you feel good about yourself.

Not in my experience.

As an NPC power, I've found Demonic Co-Pilot to be a bunch of pointless dice-rolling.

Why would you need to roll dice if the host simply accepts takedown with the first mental attack, letting the co-pilot take the wheel for the duration of the conflict?  Boom: no stress, no consequences, no rolling ... so long as the host and co-pilot have symbiotic goals.

It doesn't? Constant mental attacks, quickly ramping up to Taken Outs and Extreme Consequences that change the person into some kind of blood thirsty beast sounds pretty much exactly like what the FBI guys went through in Fool Moon.

Remember, after using the belts for only a few days before the book starts, three of them were outright addicted to it and the fourth had to hide the belts to keep them from indulging.

The problem is that after using the belt for a short time, all someone would need to do is poke them and they'd keel over, having no consequences left to sacrifice.  I still hold that a Sponsor Debt-style system would be more fun, more playable, more balanced, and better reflecting the "reality", so to speak.

Actually, I think I have to agree with Tedronai on this one; that does seem to be how the power is written.  Almost every other power I looked at is careful to specify that it grants an "ability" or that the character "may", but a couple of powers seem to deliberately not make themselves optional.  Spirit Form and Hulking Size are two others.  Of course, they can potentially be toggled on/off via (for example) a Shapeshifting Power.  As can Demonic Copilot -- but when in your "powered" form, you get the benefits and penalties of the Demonic Copilot when the conditions apply, like it or no.

Which makes the power even worse than I thought it was.

Then again, any time this is a disadvantage, you could argue a self-compel, also per the RAW.

Er ... Sponsored Magic is channeling + ritual, not evocation + thaumaturgy.  So they get the equivalent of one element and one theme, with no additional specializations (other than what their sponsored magic grants them) unless thay also buy evocation and/or thaumaturgy.