To be honest, I haven't done the math, but I'm guessing that within a certain percentage, the F4U-1D, F6F, & P-51D are all roughly equivalent to the N1K2 when taking the cannons into account. I didn't really think about the P-38 since I feel its a terrible attack fighter (against a defended target). Since it has such a vast potential ammo load for the guns and 5" HVAR capability, the P-38L may go in with the F4U-1C and bubble top P-47's. The P-47D-25 would be debatable since it can't carry 5" HVAR, but still has 3200 rounds of caliber .50.
However, and not to be obtuse, but in order to narrow the focus of the discussion to your OP and not let the thread wander, I didn't make the comparison to any British or German aircraft. You compared the N1K2 to U.S. aircraft only. Comparing the capability of the N1K2 to the Mosquito or 110 would be a totally different comparison, and one in which I feel the N1K2 would lose hands-down.