I met one of the lead engineers of the P-51. He also made a "role playing" miniatures game called Mustang and Messerschmidts. If you ever look closely at P-51's, you'll notice two seemingly minor changes to the design which had a major impact on the performance. The facts behind the changes were still classified Top Secret until 1991. The changes were...
1. The most noticeable was the "pass through" oil-cooler scoop on the bottom of the plane. This wasn't added until the P-51B model. The design change was simply intended to increase the cooling of the oil. The surprise wasn't accepted until a few months later when many P-51 pilots kept reporting that the enemy were suddenly slower. It turned out that the new scoop design actually acts like a mini-ramjet and give the plane a top-speed boost of up to 25 knots. That is HUGE for a prop-driven fighter. All for no extra fuel.
2. Later, they modified the exhaust ports on the side of the plane (up near the nose). The change was from port openings flush with the side panels to tubes angling back towards the rear. At full throttle, this "little" bit of exhaust provides up to 5 knots boost, again at no extra fuel cost.
The Germans never learned why their ME-109 was always a bit slower than the P-51 even though all aerodynamic and performance test said it should have been a bit faster. The US kept the secret closely guarded while other countries continued to use P-51s even into the 1980s.
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