While true that combat victory stats are very difficult to verify the "top aces" are interesting case studies and they have been indeed studied:
The wiki seems to show quite good analysis of Johnie Johnson´s record:
https://en.wikipedia.org/wiki/Johnni...n_(RAF_officer)
Similar analysis seems to have been made on the claims of Aleksander Pokryshin:
https://en.wikipedia.org/wiki/Alexander_Pokryshkin
At least both men seem to fit very comfortably to "verified ace" category! (with more than 5 air victories confirmed or likely by enemy records)
Gabby Gabreski was the top US ace in Europe, but I wonder if they have made any studies to check his record? I guess that Gabreski flew more combat missions than "average" USAAF fighter pilot in Europe and even after critical checking of his air victories Gabreski will likely remain as a top US ace in Europe during WW2...
A minor case but perhaps interesting: Pavel Kutakhov (long time C-in-C of Soviet Air Force) was credited with 13 air victories (individual) + 28 shared (!) in WW2. IIRC, of his individual victories 5 could be verified by German records - so he did make it an "ace" but perhaps with a overclaiming ratio of some 2,5:1 (+ shared ones). Rather "normal overclaim" ratio in WW2?