I think that some point to the Spitfire's max dive speed as being a result of low drag. The Spitfire had a significantly higher drag coefficient than the P-51. Its wing, however, had an excellent low chord to thickness ratio, delaying the onset of compressibility. On the same horsepower, the P-51B was more than 30 mph faster than the Spitfire IX.
A Spitfire PR recorded Mach .91 in a dive. However, that would not have been possible, except that the prop shaft failed and the propeller came off. Indeed, a major limiting factor in prop driven aircraft speed is t
[/img]he propeller itself. Generally speaking, at very high speeds, it's little more than a circular speed brake. The issue for Spits in combat was not maximum velocity, but the rather leisurely dive acceleration rate (in comparison to the P-47 and Mustang).
Test pilot Herb Fisher more than a hundred test dives in a P-47D, testing standard and high mach propellers. His P-47 would reach a terminal velocity of Mach .81 with the standard Curtiss-Wright prop. Using the lower drag designs, he could manage Mach .83. Ultimately, that was terminal velocity for the P-47. Critical Mach was .73, whereupon a shockwave would begin to form. Light buffeting would begin at Mach .71 and full boogie shake at critical mach.
