The P51 was a brilliant aircraft,the Merlin[licence built by Packard]one of the best piston aero engines of them all.It had range to escort the "heavies" on their deep penetration raids and the manoeuvrability to hold its own against Germanys best.Still in use in the Korean war I believe?Armed with the .50cal that the yanks seemed so wedded to for such a long time.
Yes, they made all the difference to protecting the US 8th Army B17 bombers all the way to Germany and back, the first fighters to do that. Their belated introduction, after intitial hesitation by the US Command, stopped the horrific losses seen up to this time of B17's and their crews during their infamous daylight raid policy. Only the eventual introduction by the Luftwaffa of the ME262's in significant numbers in late 1944 / early 1945 became a new threat, but by then it was too late. Only 300 ME262's were commissioned out of 1400 built, and by then there was a grave shortage of fuel, parts and pilots within the Third Reich, and many were just abondoned.
Yes, the Mustang served in Korea, and I once knew an American friend who flew these as well as the jets that took over during that war.