Well, the wing area and weights (wing loading) are the most imporatant indicators of turn performance and energy retention in a turn, and also the h.p. of the engines.
If you wonder why the A6M2 has a such great turn ability, all you need to do is calculate its loaded weight and wing area. Same for a Spitfire, or a Ki-61, or an MC202, or a Bf 109F.
Interesting info WW, but those are generalised statements at best, it would depend on specific loudouts.
The FM-2 also benefits from having a more powerfull engine (1350 vs 1175) where the Ki-61 has lower drag (inline design), probably cancelling each other out in a sustained turn contest.
So:
Weight: FM-2 lighter.
Wing area: FM-2 larger wing.
Engine: FM-2 more powerfull.
Thats why its as competative as it is.
PS: You dont have to blow a gasket dude, Im just posting a comparison.