The issue is that a lot of the testing itself was flawed. The placement of the air intake for airspeed had a flaw in it and was later moved, but when flown side-by-side with an F4u corsair, it matched its speed almost identically. The test pilots thought there was no difference in the plane performance, other than the F6F turned a little tighter, and one of them (F4u?) climbed a little faster.
But, then, they were flying the real deal, and did not have Aces High Brand (TM) uberflappen. Excessive use of flaps in this game totally throws the F6Fs manuverability into the trash bin when compared to the flaps-down F4u in-game.