From what I've read, the only large bore, short range (in comparison to high altitude flak guns) that were even remotely successful against aircraft were the 40mm Bofors mounted on gun carriages and M-19's. M-19's had double Bofors, helping it considerably.
The M-15a1 had a production run of 1,500 but was considered very ineffective because of tracking problems and was considered more useful as a infantry support vehicle with the 37mm and 2 .50's (!)
M-16's delivered lots of lead per second and were very effective in the AAA role and infantry support.
The Pz-38 flak, and the German halftracks mounting single 20mm AAA guns had much more in service and were said to be more effective.
Whats really sad is that the N1K2 may of had a 2000 HP engine but due to fuel and the manufactering problems with the engine due to the B-29's bombing, they had lots of problems even getting them to work right, let alone produce peak power. Random engine failures should be modeled!

Least it isn't the N1K1-Ja. Awful hard to land that one without damaging it's telescoping gear.
- Jig