Its easy to get hung about about the numbers when sometimes it pays to stand back and look at the problem from a different point of view.
There is no doubt that the USA aircraft armed with 4 or 6 HMG's were more than sufficient for the task they had to face. They may not as been as efficient as a 20mm armed aircraft, but they were efficient enough for the task they had to do.
For the USN they had slightly different requirements compared to the USAAF. If a USAAF aircraft ran out of ammunition they could head for home, if home had been attacked and damaged it was pretty certain that they could divert or in a worst case scenario crash land or bail out over friendly territory. This is untenable for a USN aircraft, it has to carry on fighting. If 4 x HMG is good enough to do the job, then there are good reasons for the USN to accept 4 guns and more ammunition.
To critisise the Wildcat for not being equipped with 20mm from the start is being more than a little mean, 4 x HMG in late 1940 is above average for a single engined fighter. Certainly better than the 8 LMG on the RAF fighters, at least as good as the Me109 and by some margin better than those 109's only armed with 4 x LMG.
The ROF argument is also I believe exagerated. The 0.5 M2 fired about 13 RPSecond, the Hispano II 10 RPS. If you cannot hit someone with 10RPS, then you are still likely to miss them with 13RPS.
Instead of getting worked up over fractions of a MW here or there I find it easier to keep it simple. The USN compared the 0.5 to the Hispano 20mm and decided the 20mm was three times more effective, the USAAF did similar tests and decided that it was 2.5 times more effective. So a Hellcat, Kittyhawk or a P51D is about equal to Spitfire as the 4 x LMG often carried were of little use in Europe and ignored. If we are talking about the Pacific then I would give the edge to the Spitfire as the LMG's could do damage to the average Japanese aircraft.