A few facts are needed as the flight model has changed since the 2.07 update.
Among mid and late war American fighters, the P-51D currently has the largest turning circle and nearly the lowest turn rate, with full flaps. With two notches of flaps, the turn rate increases without a big difference in radius. My advice is not to use full flaps. The P-51B is slightly better, but not significantly so.
Against the La7 or Bf 109K-4, either P-51 is now easily beaten in a turn fight. Inasmuch as the La-7 will run down, out-climb and out-turn either Mustang, you better be one hell of a pilot. Likewise, the 109K-4 will out-cllimb and out-turn the Mustangs. Both the La-7 and the 109K-4 accelerate much faster. Things being as they exist after the update, given equal pilots the P-51s will lose. Indeed, the La-7 will beat the 109K4 under the same criteria. Since the update, the 190D-9 can now afford to dogfight a P-51 as it can match them on turn radius and actually maintain a higher turn rate.
There's only one way the Mustangs can have and maintain an advantage. Fight all duels at 20,000 feet or higher. Up there the P-51 can win.
My regards,
Widewing