Oh and as for actually flying/fighting in the P-51:
First of all, and most importantly, you do not HAVE to be the highest/fastest plane around. You do not HAVE to always stay fast. Flying like that will only cause you to be timid and an excessive runner, which probably describes most of the P-51's you encounter in the MA.
Avoid very low altitudes if you can in a P-51B because, frankly, it sucks in the weeds. Its engine is tuned for high altitude flying. It isn't completely helpless on the deck, but is decidedly out of its element. Terrains with a high "deck" level, like Mindanao, or Pizza or parts of Trinity, tend to be good for the P-51B. staying above about 5K will do much to keep you alive in a P-51B.
Properly loaded, the P-51 is perfectly capable of dogfighting against things like the 190's, F4U's, and Yak-9's. LA7's tended to beat me more often than not in a slow fight under about 7500 feet--but I had a bad habit of running into the better LA7 pilots.
There is over a 600 FPM in difference between a light and a fully-loaded P-51D in climbrate at low altitude, not even acconting for external stores. The P-51B is equally as weight-sensitive.
Do not overload the Mustang. Flying around with 75% fuel, full ammo, and rocket tubes (in the case of the B) will result in you being a SLUG. A light enough P-51B can hold its own against even things like P-38's and F6F's, but Spits and N1K2's will always eat you alive in a dogfight. Be more aggressive as your plane gets lighter. I would avoid doing much slow fighting if I had more than about 75% fuel in my wing tanks or ANYTHING in the AUX tank. Always, ALWAYS drain that AUX tank first, even if you have droptanks. Even a little gas in that tank can mess with the stability of the P-51 and it rapidly becomes annoying in a dogfight.
Always maintain good SA, but that is true for any aircraft. Do frequent S-turns to check your rear blind spot in the P-51B.
In the P-51B, be a good shot. 4 guns are adequate, but you don't get as much ammo as you do in the other American fighters. I always liked the B because it is a very stable gun platform, noticably more stable than the -D.
J_A_B