Came across this comment from a Mustang pilot while looking for something completely different regarding the 364th FG Mustangs. I'd suggest this guys memory was fairly good at the time as he was being interviewed by the head of the National Air & Space museum. He's been a two tour combat pilot in WW2, retired as a 4 Star general, leading the Minuteman Rocket program as one of his assignments, directing the NASA lunar landing program and also head of NSA at one point.
Posted purely for the insight, not to get anyone worked up. I just thought it interesting. General Samuel Phillips His group started to transition to 51s at the end of July 1944:
"Fairly quickly after the appearance of the ME-262, our Air Force's counter was a fuel additive. The fuel that we'd been using was 100 octane, and I remember the maximum power setting, military power in the P-51 was 3000 RPM and 65 inches of manifold pressure. I've forgotten exactly what it gave but that's somewhere in the ballpark I guess of 2000 horsepower on that engine. At full military power the P-51--I'd have to go back and check to get numbers--but at low altitude the air speed had to have been somewhere getting up around 300 miles an hour, but it was still well short of what the ME-262 could do. So with the fuel additive that was provided at our bases fairly quickly, we could go up to 85 inches of manifold pressure, and that increased the horsepower output of the engine by a lot. One of the penalties was that the engine had to be changed after each mission, because it would virtually burn up the engine to run it very long at those high powers. But the speed difference was not so great. The P-51 chasing an ME-262 couldn't catch it. In other words it was short of the 262 speed. But there was more than one ME-262 that were shot down by P-51s in that period. I don't know the statistics, but I guess I was impressed in retrospect with the speed with which fuel additives were provided, so the knowledge of how to do that had to be there. And with the performance increase that that provided. But it was still well short of the jet engine"