In 1941, test pilot Bob Fausel broke the world speed record in a power dive in a Curtiss P-40, reaching 661 mph.
The P-40 was a very robust aircraft; a lot of aircraft of the time would have never withstood those kinds of speeds without structural issues or a loss of control.
Observers regret that the engines could follow. The P-40 was largely a P-36 with an inline Allison engine replacing a bulky radial engine. All talk about the Allison, always redirects to the lack of a decent supercharger for high altitude
The Me-209 and He-100 had it beat by a long ways in level flight. But the high diving speeds did pay dividends in combat though, especially with the Flying Tigers.