Hey, Nofly. It would be awfully nice if you knew what the hell you were talking about. Most important airplane in history!!? Are you kidding me? The air war would not have been won without it? The early Mustangs weren't "duds" and it just so happens the British DID like it. They used them at low level for, what they called, Army cooperation missons. The USAAF called it Tactical Reconaissance. At sea level, it was one of the fastest airplanes in the world. That's with the Allison engine.
They liked the airframe so much, in fact, that they thought it was worth trying out a Rolls Royce Merlin in it.
The airplane was great, but could the air war have been won without it? Yes. Because Republic designed the P-47N, and had there been no Mustangs, those are the fast, long range fighters that would have wound up escorting 8th Air Force bombers to Germany. Instead, not needed in Europe, P-47N's flew long missions to Japan from Ie Shima and Iwo Jima. VLR missions...very long range.
And the P-51's used during the second world war had a top speed of 435 MPH, not 455.