I'm reading a book about Guadalcanal and it has some interesting points on the air war there. One was that some pilots preferred more ammo for four guns than less ammo for six, so they disabled one in each wing.
You're quite right on kills does not equal aircraft effectiveness. Zeros sacrificed armor for performance and kicked ass to begin with. Once American pilots adjusted tactics, they felt they evened the odds or pushed them in favor of USN/USMC fighters. Check out the Thach Weave.