Don, I have been aware of the issue you wrote above for some time, but I didn't mention it here, because I thought it would be easier just to use the maximum speed, and also I didn't want to add more stuff on your plate. In some cases speeds at sea level can be easily found, in others not. In addition, if we want to be really nit-picky, external stores usually limit the speed even more, since they add significant drag.
Reviewing all the aicraft speeds is a big task, especially if absolute maximum speed can't be used. I just did that for the USMC OOB, but didn't post it here yet, and I used maximum speeds. Now, I can
try to re-do it at sea level speeds if you want to. Joe Baugher's American Combat Aircraft (
http://www.joebaugher.com/uscombataircraft.html) is a good source, since it has all USAAC/USAAF/USAF aircraft (unfortunately only some USN) and it usually lists speeds at several different altitudes. Unfortunately I do not know any on-line sources which would do that consistently for British or German aircraft, let alone Russian, Italian or Japanese.
Oh, and the P-47M actually could do 756 km/h (470 mph) at 9,000 meters (30,000 ft)
