Haven't seen the film yet, and this may be in it. But how many of you knew that the Mustang was built for the RAF originally? It was with them that it first saw action, and I suspect that the British had the idea of hooking up a Merlin engine to it in lieu of the Allison, letting it reach its full potential.

However, some Allison-engined Mustangs served well over Burma, with the US First Air Commando Wing. They didn't as often fight at high altitude as over Europe, and the Allison engine usually sufficed, although it fell off badly at much over 14,000 feet.

A relative few P-40's (the P-40F) were given Merlin engines, but I can't find anything about how well they did compared to the Allison-engined ones. Some P-40F's were used in Operation Torch, the Allied invasion of North Africa.

T-Star