I agree with GRUNHERZ.
Giving the USN both the FM2 and F6F-5 gives the USN the one-two punch against the IJN/IJA. The FM2 can nearly turn with the A6M5b and will out turn the Ki.61-I-KAIc. The F6F-5 is faster than either, and is only threatened at high speed by a diving Ki.61-I-KAIc as the A6M5b can't follow the USN fighters in manuvers at speeds above 300mph.
The F6F-5s will force the Japanese to dodge, the the FM2s will keep them slow while the F6F-5s finish them.
It'd be like giving the US the F6F-5 and the Japanese the A6M5b and the Ki.84-Ia (if we had it). The F6F-5s would get slaughtered because they couldn't run or climb with the Ki.84s and the A6M5s would keep them from living by turn fighting. The whines, justifiably, would be deafening.
Don't predetermine the victor by handing one side an unbeatable (realistically) fighter combination.