They're very close Ouch, below is using the P51D best speed vs Altitude:
P51D at 12,000 feet, WEP on:
420 mph
P51D at 25,000 feet, WEP on:
440 mph
Tempest at 12,000 feet, WEP on:
390 mph
P51D at 25,000 feet, WEP on:
440.
So, if you were below 15k but above 5k, then he had the advantage, however if you were at 5k, then your advantages were equal.