With regards to the Bell P-39 i have to agree that just shoe-horning a Merlin engine into it would by no means turn the aircraft into a better fighter. The P-51 had to be quite extensively modified to accept the Merlin, and as the P-39 was mid engine im not convinced this would have been the answer. I also think the Allison engines have gotten a bit of a bad rep over the years. And i also agree that i dont think the problem lay with the engine itself but more with the superchargers/fuel/air mixture set up.
The main work involved with the P-51 change was increasing the radiator capacity.
The conversion could have been done more simply, as Rolls-Royce had done with the Mustang X, but NAA did more optimisation of the installation.
The P-39 would not have taken the 2 stage Merlin.
The Allison engine was underdeveloped at the time. The supercharger wasn't well developed, and they suffered from not developing their own earlier (originally the supercharger impeller was supplied by General E
The unpredictability of a dogfight doesnt always mean one aircraft can be labelled as inferior even if it does perform badly at altitude. Most pilots had to fly their birds to the best of their ability and utilize its own strengths and avoid weaknesses. No half sensible pilot would have gotten involved in a dogfight at altitude in a P-39. Just like P-40 pilots wouldnt get into a turning fight with a Zero. I guess its horses for courses. Some aircraft were liked by pilots and some were not. I do believe some/at least one fighter squadron based in England actually gave up their Spitfires/Mustangs to fly P -47s instead of the other way around which is what a lot did. Although most accounts i have read state some units were very much against changing from an agile Spit to a huge and heavy workhorse like the P-47.
Tests were done to aircraft to determine where they performed best.
The P-39 was seen as inferior by the RAF because it performed poorly at the typical altitudes at which they fought, particularly for intercepting bombers.
My choice of forgotten/underrated aircraft would have to be the Hawker Hurricane. Always linked to the battle of Britain and perceived to be obsolete/out of date by the end of 1940 i feel the Hurricane has had its achievements overshadowed by the more glamorous Spit.
The Hurricane was obsolete as a fighter by the end of 1940, if not before. They still kept making them, however, as having an operational aircraft was preferred over having a superior aircraft in design or development.
The Tornado/Typhoon twins were supposed to replace the Hurricane and the Spitfire by that time. But it was nowhere close, and, in the end, was incapable of replacing the Spitfire at all.
The Hurricane was relegated to secondary theatres and/or secondary roles until its retirement.
In the early part of the war, in particular, production was king, and that's why the Hurricane continued as long as it did. If there were sufficient Spitfires at the end of 1940 the Hurricane probably would have disappeared sooner.