"The XP-47M proved to be nearly as fast as the XP-47J. 488 mph was obtained on at least one flight. The official maximum speed is 470 mph. However, over-boosting the engine could tweek another 15 to 20 mph out of the big fighter. There is adequate evidence to indicate that some of the more resourceful crew chiefs in the 56th Fighter Group, managed to hotrod the P-47M to the point that some reliable pilots were reporting 500 mph at altitude in level flight."
P-47M
The P-51 and P-47 were designed to escort high altitude bombers, the Tempest was optimised for low/medium level. The P-51H reached 487 m.p.h.at 25,000 ft. The Tempest V reached 435mph at 17,000ft.