The P-38 performed well in North Africa, Italy, and the Pacific as the combat was a lower altitudes where the air temperature was warmer and compressibility was not an issue in high speed dives from lower altitude.
This article includes reference to the P38 as a new fighter, whereas the British had already toyed with the idea of buying them in the Battle of Britain timeframe, but blundered by insisting on saving money, pennywise and pound foolishly, by not buying the most important feature, its turbosuperchargers - which are hard to beat at high altitudes in particular.Any way you look at it, having an advantage in high altitude performance is never going to hurt a pilot’s feelings - that just naturally gives him the ability to “boom and zoom” - to dive at high speed, take a shooting pass, and rapidly escape back to high altitude from which to dictate the terms of engagement, or retreat as the situation seems to demand.
The speed of sound varies with temperature, and the P38’s advantage in high altitude/high speed worked better in a warmer climate where the Mach effects which hindered the P38’s early design was not as much of a constraint in the Pacific as in northern Europe. The upshot is that as a general rule all P38 production should have been diverted to the Pacific immediately after Pearl Harbor. But at the time, it was politic to send them to Europe until Doolittle shook things up.
Another easy-to-overlook aspect of comparison between fighters is brute simple: price. It’s obvious that P38s must have been more expensive than most or all American fighters. Twin engines alone would tell you that - and turbosuperchargers as well. But among single engine fighters, there were significant differences as well. The P47 was a big plane for a fighter, but it was able to perform at high altitudes - again, a turbosupercharger will do that for you - and the testimony of high-scoring ace Bob Johnson was that its roll rate could be a significant advantage. Bombers wanted high altitude to be further from German FLAK batteries, which explains why so much of the air war over Europe was at high altitude. So, within its range limitations, the P-47 was quite good. The other factor was pilot survivability; heavy construction, armor, and a smooth belly to make belly landings less dangerous - plus an air-cooled radial engine less vulnerable to a single gunshot - meant that not every P47 pilot was eager to switch to anything else.
But the P-51 came along in 1944, and was not only a stellar performer but it was notably cheaper, including the mechanical drive supercharger. It became the star of the show, but that was partly because of the attrition the Luftwaffe pilots suffered fighting the P-47s in 1943.
In the Pacific, the Corsair had huge teething problems despite excellent battle success. The Hellcat wasn’t as fast a plane, but it was dominant over the Zero, which was good enough - and it was something like a third less expensive. In addition to being much simpler to land on a carrier.