Just about the same airframe, but with a much more powerful engine, I'd assume the horsepower makes the FM2 dive better.
The weight of the guns did reduce performance a bit, but I believe in past discussions the USN felt the added firepower made up for that difference. Also remember the wings had a wing fold mechanism that also added weight. So overall the 6-gun, folding wing, f4f-4 was a bit heavier than the 4-gun, non-folding, f4f-3.