You just need to spend more time in the P-51 and get to know its strengths.
The AI routines differ for each plane slightly (I believe Oleg had confirmed this sometime) and for some reason, the AI with the P-51 is particularly "stupid" - not attacking when the enemy on my six is right in front or him, for instance.
Using Il-2 Compare, the D is the fastest of the group, but not by much. The "problem" is that it's slow at accelerating so you have to be especially vigilant to not bleed off your speed. Try to fly it less like a Bf-109, and more like a P-38. Having a team-mate is very important in case you get in trouble.
I have been able to outrun almost anything in a P-51 if the trim is good. The only thing to give me a hard time (at low level btw) were La-7s and the dreadful I-185 type 71. I've outrun Spit25s without problem and have caught the "uber" Dora.
To me it seems that because the P-51 was on the winning side of the war, that it was some wonder-machine. People love to cite how Goerring apparently said something along the lines of, we've lost the war, after seeing P-51s over Berlin. I feel he would have said the same thing (if he ever did) if those were P-40s or Spitfires. It didn't matter how good the P-51 was - all that mattered was that the daylight bombers now had an escort for their entire trip to and from Germany, making it just that much harder for German pilots to shoot them down. That's it!
It doesn't have to be un-breakable, pull fancy maneuvers, tear wings off by lobbing huge cannon rounds - it just has to be fast and keep the interceptors at bay so the bombers can do their work.
Also, keep in mind by the time the P-51 was around, the Axis forces had already lost many of their best pilots and new recruits were being pressed into service with very abbreviated training. Allied pilots, especially American, had the luxury of a full-blown training cycle, and so were better off from the start.
There's this sort of "cult of personality" about the P-51 that I wish didn't have to be. I feel that it's causing this disappointment when people first fly it, and realize it's not a La-7.
All that said, I feel it's great online if flown carefully. (Thor's video is proof of that!)
I've not flown a real Mustang myself, but someone who did has said that while it's fast as it should be, some aspects of the handling are not quite the same.
It would be interesting to note that a particular Brewster Buffalo has more kills attributed to it than any other airframe used in WWII
