The main thing the P-51D had going for it was its long range. It was the fighter that took the war to the Germans. That is undisputed.
I too think the P-51D is overated but not because it was not a good plane. It was one hell of a plane. The reason I think it is overated is because when someone says what is the best plane ever buit and automatically the majority of people go "P-51 Mustang!".
You have to look at it this way. Each aircraft had ist point where it was operating at maximum efficiency and ability. Some aircraft that was at high alltitude and some it was at lower alltitudes.
The P-51D would be superior at certain altitudes to other aircraft and not superior at other altitudes.
The P-51D I believe however was superior at the altitudes it needed to operate at and that was where the bombers were. Dont quote me on this though.