As someone said earlier, a British engineer noted the Rolls-Royce Merlin 61 was almost identical in size to the V-1710. As such, the British decided to put the Merlin 61 into a slightly modified P-51A airframe (the Mustang X) and the British test pilots were shocked it gained 10,000 feet in maximum altitude and the top speed went from 387 to 432 mph! The results were so impressive that North American decided to build a P-51 version designed around the Packard-built version of the Merlin 61, and the resulting plane had a top speed of 441 mph when tested in late 1942! (In late 1942, the P-51B prototype at 441 mph was easily the fastest fighter in the world, no contest.)
So what, specifically, made the Merlin engine so superior to the Allison? Horsepower is made by superior breathing/compression so, obviously, the Merlin had bigger intake/exhaust ports/valves, but did it have more exotic (for the time) refinements i.e. multi valves, DOHC (chain driven or belt), mechanical fuel injection vs water injected carbs or exotic materials like newer aluminum alloys vs magnesium? But having achieved superior performance in piston driven aircraft the push for additional performance gains died with the advent of jet power by the Germans.