What Karnak said, J2M3 was built for speed and climb, as for 20mm in the fuselage, well, the inline engined Ki-61 has them, so its hardly that surprising.
It was a bomber interceptor, in AH, it would likely be something of a hanger queen, being that the N1K2 has a lower wing loading and also climbs fast. Both the Ki-84 and N1K2 are better vs fighters than a J2M3 Raiden.
From what I have read, many IJN pilots didnt like it, as it could not manuever as well with US escort fighters.