Without forced induction you won't see any significant power increase, and if they're claiming increases without boost then it's likely bogus. The P-51 had a two-stage supercharger, the NA meant "North American" who was the manufacturer of the P-51. Unless you have something to force additional air into an engine then the volume of fuel available to generate power is pretty much limited by displacement. But like P. T. Barnum said, "There's a sucker born every minute" and it seems most own vehicles.