Yes, the F6F is too slow at altitude. Has this always been the case?
Yes...
Should easily top 400 (406-409) mph at altitude (around 22k). It does no better than 386 mph. Heck, the F6F-3 could do 391 mph.
Speed below 15k (or there about; I have the data in my notebook) is correct, or close enough.
There's Navy test data showing 393 mph at MIL power (no WEP) and around 408 mph with WEP. Just remember that WEP lowers the critical altitude some.
The reason it has little effect on game play is that few F6Fs will be found above 15k.
My regards,
Widewing