I've heard Oleg say that the most scientifically sound measurement of the P-51 gave it a top speed of around 408 mph.
Yet he is choosing to use higher numbers from NACA and USAAF in FB. I don't think he is yielding to the Mustang Mafia in this case. The problem has to do with relative airplane performance.
The Spit IX used nearly the same engine as the P-51B, and is said to have maxxed out at about 411 mph. When the RAF 1st began using P-51's there were many occasions were Spit IX's and P-51B's flew missions together. The pilots noticed that at all power settings the P-51 was faster than the Spit IX. So if the P-51 was really a 408mph plane, how fast was the Spit IX? And if the Spit IX was really a 385mph plane, how fast was the Bf-109G-6? Pretty soon you have to lower the speeds of all planes in the FM to keep their relative performance in line.
If NACA and the USAAF came up with similar numbers, then those numbers are probably reasonable. The RAF test may have been conducted on a less than optimum aircraft.
ra