I would have imagined level flight to be the discerning factor here.
After all, the Spitfire was involved in a lot of test flying with terminal dives to discover mach limiting speeds etc. and got over 600mph I was once told. The pilot, a Sqn Ldr Martindale, had the prop assy detach in one of these dives and still managed to get it back to Terra Firma. Brave man....
Conan