In Aces High, maximum speeds are relative to altitude. Above 20k the P-51s are faster than the Tempest. At 30k, the Jugs are faster too. Don't forget the Bf 109G-10, which flirts with 450 mph at its best altitude. Even the P-51D can reach 441 mph at 25,000 feet.
Yeah, the Tempest is a monster on the deck, but at 20k it accelerates like a bus; easily beaten by many fighters in service by 1943, including the P-38G, Spitfire Mk.IX and the Bf 109G-6. Even the La-7 accelerates faster at 20,000 feet.
There are two fighters that I prefer to the Tempest; these are the Spitfire Mk.XIV (444 mph at 28.5k) and the F4U-4 (446 mph at 25k). By the way, those are actual measured speeds for AH2.
My regards,
Widewing