OK, here are the full details of why I suggest a6m5's in the mix.
The US has these non-historical advantages based on what planes are available (as per
http://www.ww2pacific.com/aaf41.html):
No Brewster Buffalos, only F4F-4's.
No Curtiss P-36 Mohawks, only P-40B's.
No Boeing P-26 Peashooters, only P-40B's.
No B-18 Bolo's (SBD-5's and TBM-3's instead, both faster, more maneuverable, harder to hit, better offensively, and likely sturdier)
No PBY's Catalinas (same as for B-18, only moreso)
No B-17D's (only B-17G, which has tail gun, more defensive armament, more armor, better self-sealing fuel tanks)
No SBD-1's (only SBD-3's, which have more armor and better self-sealing fuel tanks)
No TBD's Devestators (only TBM-3's, which are faster, more armor, better self-sealing fuel tanks)
So, the US has a lot of aircraft that are better than historical. Specifically, they are faster and sturdier than their historical counterparts. It seems to me, rather than screwing with numbers (which isn't any more historical than some other adjustment) the best way to adjust is it to give the IJN in some of its planes a little boost in speed and amount of ammo. The a6m5 isn't much better than the a6m2 -- it is a little faster and it has more ammo -- just the thing needed.
Now, if you think the IJN will cream the US even if it has all a6m2's, then I'm all for just a6m2's. However, I think the IJN will have its hands full and that the fight is not going to be so lopsided.