Except the Americans didn't run the Merlin XX/V-1650-1 at 1460hp for some reason. They ran it a 9lbs of boost for 1240hp at 11,500ft and 1120hp at 18,500ft. At least officially.
Speed difference is marginal at times and may even flip flop at certain altitudes between certain models. British data card for a Martlet/Wildcat V calls for 332mph at 21,000ft at "mean" weight. This may very well be high but most F4F-4s should be good for 315-320mph at around 20-21,000ft. Speeds quoted for Sea Hurricanes IIs are all over the map, from 314mph to 342mph. Why the Sea Hurricane with 4 20mm guns and carrier gear should be faster at 21,000ft than a MK IIA land plane with 8 .303s I have no idea.
F4F-4 was tested as faster than a P-40F above 22,000ft. Bad P40-F???