Not looking for trouble here as a "junior member" but I think some of Foss's 26 kills were bettys/other ducks (I know you guys are aware of this already).
The fact that he did have at least "many" zero kills while flying a wildcat does tell you something though. I imagine there were tough times when he would have traded in his wildcat for a spit. ...
Yes some of his victory credits were bombers and other (eg. floatplanes, Foss himself was once shot down by the rear gunner of a Type 0 Observation Seaplane, aka 'Pete'). Moreover at that time of war Japanese losses were typically around 25-33% of what Allied pilots were credited with, not as accurate as in some theaters and periods in WWII (eg., it's not fair to compare that score *numerically* to ones credited to later Allied pilots under stricter rules ca. 1944-5). But relatively speaking Foss was an outstanding pilot, and at least as importantly, an outstanding leader. The overall accomplishments of F4F units in 1942 against Zeroes were better than anybody else's in that period of the war and Foss was a major leader at a critical time.
On the second statement this is the whole point of the Spit/Darwin discussion, when compared to the tactically similar F4F/Guadalcanal situation. The F4F's did a *lot* better, not a little better, both as measured by Japanese accounts. Seems hard to argue that the Japanese somehow decided to 'understate losses' more v Spitfires than F4F's (though there's no evidence they did so in either case, no more evidence than that the Allies did, ie. no evidence

).
F4F's in 1942 had an exchange ratio of just about 1:1 in fighter combat v Zeroes (on a scale of 100+ each), a little better considering just the G'canal actions. Spits had a ratio of something like 1:5 against Zeroes (and Oscars) over Darwin in 1943, combat losses only. Since it's a high ratio, of course it would mathematically change relatively a lot with one or two more Japanese losses (besides the 4 Zeroes and 1 Oscar apparently lost in all those raids), and it's a smaller sample (5 known Japanese fighters ~25+ Spits combat only) but no way close to 1:1.
Would Foss have eagerly traded his F4F for a Spit V? Would a modern observor who focuses on theoretical paper performance have done, probably; would Foss have?, I don't think that's so obvious actually.
Re: 'myth' it's always relative to what the conventional wisdom is. I think there's also a myth among some that there was a simple easy answer to combating Japanese Navy fighter units, and once the magical formula (boom and zoom, or other oversimplified description) was adopted the Zero turned into a pumpkin. I think the Darwin example shows the fallacy of that. Certain defeats of US fighter units in the Solomons in 1943 also show it. The Japanese Navy fighter units gradually declined from just about unbeatable (first 1/2 1942) to parity (from mid '42 w/ the best Allied fighter units, more generally by mid 43 with exceptions like the Spits at Darwin); to definite Allied advantage (from mid 43) to sometimes completely overmatched Japanese Navy units (which didn't happen often until 1944). There was no sudden bursting of a bubble.
Joe