The Me 109 G was red lined at 466mph whereas the Spit IX was redlined at around 455mph so the difference wasn't great but the 109 in a dive accelerated faster so had a tactical advantage greater than the actual numbers indicated. Its a short term gain as Terra Ferma will be reached sooner or later.
However the P51 and P47 were redlined at speeds in excess of 500 mph so the 109 was at a significant disadvantage when trying to dive away.
I am trying to dig out the exact numbers but this is a rough guide
I don't think so - from everything I have seen the dive 'limits' for the aircraft were pretty theoretical, and were more of a rough guess and a telling sign of the manufacturer's conservatism than anything else.. AFAIK, no serious terminal dive tests were even made until 1943 with most of the major types, and that was only done after report of odd behavior as combat pilots started to reach into high Mach numbers... the 109 for example - ditto the 190 - had it set at 750 kph IAS (all models from E/F/G and perhaps the earlier ones too, even though you would think there would be some difference? the 109K was set at 850kph/527mph, although again I doubt this would be largely due to some kind of improvement, rather than increased confidence that there would be no trouble.) To put it simple, the Diving Limits set were largely
guesswork.
Also, regardless of the dive speeds, from every single extreme dive test report I have seen, none of the 'big names' were truely controllable above around ca Mach 0.75. Compressibility effects were begun to be felt. There was no exception. Above that speed, if even reached in combat, the aircraft were diving uncontrollably until they reached lower altitude, and one of the pilots decided it was enough, and pulled up - or pancaked.
Personally I think, and here comes the
human factor which on a discussion board may be neglected, the biggest limitating factor on dive speeds was not on a place card riveted to the cocpit wall, but between the pilot's legs... its easy to read reports, but you are reading a pilots account, you may get the full horror of a vertical dive in a fighter plane at r
at 700-800 kph. Most were simply convinced that they were going to die - the airframe was cracking, and they knew when something goes wrong (and when it does, they knew that they will just die, there was no chance or time to get out), blood was pouring out of their ears as your eardrums can't take the rapid descent etc. It took a human being to the extreme limits of his phychological and physical endurance - most of the time, the plane could take a lot more..
