Does it stay completely hard until it reaches that temperature, or does it get progressively softer as it gets hotter?
I don't know much about powdercoating's high-temp characteristics, but f it is like most epoxies I've dealt with it starts to get softer as it reaches or exceeds the boiling point of water. If that were to happen in this case, vibration and magnetic fields pushing and pulling would be able to eventually squish the powdercoating out of the way of windings rubbing against the edges of laminations/etc., and that would probably be bad.
I guess it depends on the hardness of the stuff at high temps.
Also, how well does powdercoating stay on edges before baking? If it is like most stuff, edges will have the least coating, and that is the most important place to be heavily coated if you're wrapping wire around stator lams, for instance, if it were replacing the normal plastic or paper/etc used to keep wire from scraping/scratching/rubbing on those edges.