The P-51 and A36 both have a single stage supercharger that provides for a critical altitude of 14,000 feet after which performance trails off. That does not mean the airplane cannot fly higher, but that it will not have a lot of power above 15,000 or so. The British tested the design by diving a Allt full power from 15,000 and were shocked to see it live up to factory hype (breaking 500 mph). All of the tests were conducted with 100 octane fuel, which is worth noting.
