Chiming in from the peanut gallery here to say that was a really interesting post.
As I was composing it, I realized there were things I didn’t know that were internet-searchable, and interesting. In addition to the fact that the P-38 had more than its share of problems and was of limited value against front-line high altitude, high speed fighters, I learned that the engine that the Merlin replaced in the P-51, the Allison V-1710, was a V-12 like the Merlin, and had four valves per cylinder and a single overhead cam per bank of cylinders. The displacement of the Allison was 28 liters, vs. 27 liters for the Merlin. It was the same engine powering the P-38, the P-40, and other aircraft not in the class of the P-51D or the Spitfire.The other interesting point was the difference between the two engines. You would think there wouldn’t be much difference between two such similar engines - until you realize that the Merlin had a two-stage supercharger with inter cooling after each supercharger stage. The Allison didn’t have intercoolers at all - and, having an ME degree I can tell you that the absence of intercoolers is death to efficiency and power. No wonder! And the pathetic thing was that Allison objected to switching aircraft models away from the Allison engine to the much more effective Merlin. When I saw that, I experienced a shock of recognition. Of course they would object - that’s what a manufacturer would do. And people died because of it. Sigh . . .