Hi Mike - I believe the enhancement's order were mistaken. The cumultive effect of Mustang III/P-51B bomb rack removal should be 12mph, enhanced finish 8mph. The latter seems high as more than a few experiments to refnish/polish airframe and wings seldom achieved more than a couple of mph reduction. to achieve even 8mph ponts to a REALLY ratty Mustang III. That said FX 858 had a lot of miles on her by August 1944.The RAE's Technical Note No.Aero.1501(Flight) on Improvement of Performance of Fighter Aircraft Operating Against the German Flying Bomb dated August 1944 describes improvements and results for the Mustang III:
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I don't know to what extent these improvements were adopted by operational squadrons.
Also, ADGB decided to retard the ignition timing on the Mustang III's V-1650-7 engine. (ADGB/S.36777/Eng.4.).