Ironically they sacrificed the extra maneuverability, for additional protection, which was still insufficient, and did nothing to improve the aircraft's armament, which was, by 1943, woefully inadequate.
Agreed. Given the losses suffered, I can't really blame the designers for trying to increase survivalability as best they could, but it did hurt already marginal performance for marginal armor. I just like to correct the common misperception that Militarist Japan never used any plane armor or self-sealing fuel tanks out of some philosophical objection.
You raise a good point about armament, too. It is interesting to think about, when so much of the IJN air arm was based on striking hard as fast, on a strong first blow. A pair of 12.7mm machine guns might have been adequate in 1939, but not much later. The 7.7mm machine guns in the A6M were likely always all but useless, which is a problem when your cannon carry so few rounds. It is even more amazing when you remember that the Ki-43 was supposed to carry two 12.7mm guns, but due to supply shortages, many early war models actually carried only one, with the second replaced with a 7.7mm model. When I think about the poor bastards in Burma, assigned to try to take down B-24s in a Ki-43...
Late war fighters were usually designed with four 20mm cannon, if they were a new design, but by then it was too late.
If Japan still had carriers and well trained or experienced naval aviators in 1943-44, it would have made more sense to accelerate development of a shipboard version if the N1K Shiden-Kai. It was already in development and better than the Reppu anyway.
The A7M was really quite large. Would it have fit in the elevators of the existing carriers? I can't find information on it now, but I thought I read once that it couldn't be used on the 1941 carriers, and was intended to be flown from new, larger ones, or from land bases.
How about rather than introducing the A7M the Japanese switch the engine of the A6M earlier to the Kinsei that would make the A6M8 available in 1942-43 or have the Navy request a radial engine version of the Ki-61 and thus introduce the Ki-100 for carrier service?
How much effort would it take to navalize the Ki-61?