Pages 86-87 of the following RAND report
quote a 9.1:1 loss-exchange ratio between USAAF and Luftwaffe single-engine fighters to June 1944, although reading carefully it seems to be comparing Luftwaffe pilot losses with USAAF airframe losses. I do not think USAAF single-engine fighters were the only cause of Luftwaffe single-engine pilot losses, but even so it suggests a very favourable ratio. Having no safe airspace to train in must have been a bit of a handicap for the Luftwaffe, and I wonder what the effects of G-suits, computing gunsights, and 100-octane aviation spirit were.
Morse and Kimball’s “Methods of Operational Research”, written shortly after the war, mentions a fighter kill ratio of 10:1 between USN and IJN for 1943-44, by which time I think it is fair to say the small cohort of high-quality Japanese naval aviators from 1941 had mostly vanished.
Since both sources are post-war, I imagine that access to enemy records was possible (and if anyone had the patience to chase references I’d guess the US Strategic Bombing Survey might be the ultimate source for the numbers). US fighters in both theatres really do seem to have given their opponents a mighty beating.
As to the linked article on infantry training, I am perpetually dismayed by the emphasis the defence acquisition establishment in both the US and the UK continues to put on high-tech wonder-junk instead of fundamental combat skills.
All the best,