I don't know where the original figures came from for the P-80 speed
but the following is what the Air Force says in "Post World War II Fighters 1945-1973":
MAX SPEED P-80A 484.5 kn @ s.l. (556.9 mph)
P-80C 503.6 kn @ 7,000' (580 mph)
T-33A 471.5 kn @25,000'...