The USAF units which flew F-51's in Korea were not equipped with them at the beginning of the war but rather F-80's. So either F-51 or 47 would have been a conversion, but most of the fighter units in Far East AF *had* flown the F-51 a few years earlier, which is why the FEAF was better able to re-adopt that type, having also leftover spares, maintenance experience and even around 20 of the a/c still on hand in depots in Japan. Those particular a/c were used to equip the ROKAF, though they were also used by USAF pilots on combat missions (the 4 victories over NK a/c credited to F-51's June 29 1950 were those a/c flown by USAF pilots). Then a larger number of F-51's were shipped by carrier from the west coast in July, mainly stripped from ANG units in the western US. The 8th (2 of 3 sdns), 18th (all three) and 35th (39th and 40th FIS) Fighter Groups already operating over Korea used those to start converting back from the F-80 to the F-51. The 8th went back to all-F-80 in late 1950, 18th remained an F-51 group until it converted to F-86F's in 1953; 39th FIS became an F-86 unit in 51st FIG in 1952, 40th rotated out of Korea in 1951.I may be wrong but wasn't it a fact that most (or all) F-47's in 1950 were assigned to Air National Guard units? I always thought that the Air force didn't want to use the Air Guard units who were familiar with the 47 in the conflict so they went with the more plentiful front line F-51.
Re: P-51 WEP setting, 67" Hg manifold air pressure WEP was standard in 8th AF (though often exceeded in practice to at least around 70, according to combat reports), but many sources say the 7th AF used 80" on 115/145 fuel. The RAF used 81" on its Mustangs for low altitude V1 hunting. 67> 80 was found in RAF tests to add 30mph at s/l.
One source particularly for RAF v USAAF:
P-51 Mustang Performance
Joe