Depends how it is measured. If it´s on HP alone or theoretical speed or on an actual track. The ID is fast but capped at 250kph so it won´t win many races on ovals. The silhouettes are faster than the aquila but the Audi TT cup beats both of them thanks to push to pass.
Well i wouldve assumed its based on estimated lap times on a fairly standard track (so not exactly ovals ) but of course those are only my guesses
I think the best way to interpret it is that cars with the same index number are recognised as belonging to the same class by the sim. Different number different class. That's it.
Thats certainly the interpretation that leaves the fewest questions but looking at the list one has to think the general intention is for it to be an order, not just assigning each (truly unique) class a random number. And since multiclass starting grids are ordered by performance index it would make sense for it to be an order putting the fastest cars at the front and so on but for that to truly work a few classes seem out of place