Fastest propeller driven aircraft was ??????

F84 converted to turboprob with piston engine booster in tail section.

Hoped for speed was 500 knots (575 mph).
Achieved was 470 knots (540 mph). Used to have one of those crazy things up on a pedestal outside Bakersfield airport. It went to a museum somewhere and was replaced with T38.

Mind you, those speeds were achieved in level flight.

Rather imagine P72 would have run into both prop and piston engine outer limits.

Wonder what Kelly Johnson could have done with the P38 had he and it not suffered from incessant micromanagement?

Even with a Congressionally hobbled Allison 1710, could sustain 360 knots (400 mph)
for one hour.