Once a player hits 30,000 ft, he had better hope that he is in a P47x, Ta152, F4U-4, or Spitfire 14. Sure, some other planes may have the engine to take them to 400+ TAS, but they do not have the wings to allow them to perform maneuvers. Ultimately, the best "fighter" at 30k+ is the Spitfire 14. The best bomber "interceptor/killer" is going to be the Ta152. Somewhere between the Spit14 and the Ta152 is the P47M/N. The thing about the P47's are their range, they have a range that is unrivaled up that high and make them a better escort thanks to a huge ammo load and a massive amount of "keep them away" ammo. The Spit14 and Ta152 are more specialized, the P47's are more versatile.
The P51D is a dead duck above 28k, actually the P51B performs better up high in terms of engine ability. However, either of the P51's get sluggish in comparison up high. Same goes for the 190's and especially the 109's.