Actually I have been doing some research on the P-51 and too my somewhat surprise the real plane was not the best dogfighter out there. The game is looking more and more accurate. Its best virtue was range as an escort fighter while having excellent speed and decent maneuverability.
I read the actual flight manual and as many actual flight reports as I could find. The game's flight dynamics seem to match. The airplane had some real vices. The stall charactaristics ("spinning in") were not forgiving, especially if full of fuel and/or flying under 180 ias. Flick maneuvers were prohibited due to the tendancy to stall the opposite wing - which causes a spin just like in the game. At high altitude no aerobatics recommended. The airplane did not have a lot of power and did appear to lose energy quickly from what I read. It took a long time to reach cruising altitude and speed.
This graph gives a good idea what to expect.
http://www.zenoswarbirdvideos.com/Im...51/P-51OLL.gif
If I am reading this correctly, the p-51 will power on stall at 200 ias if you pull a little more than 4gs, but it takes 8 g's to stall the airplane at ~250 ias. So keep it fast and you can use much more of the airplane's maneuvering envelope.
For range comparison, the La-5 only had about 40 mins of flight time, had wooden wings, and very low ammunition for its 2 cannons. Its a pure T&B fighter designed to provide air superiority at low altitude over a battlefield.
The p-51's virtues (aside from fuel economy/range) were handling at high speed, good diving speed, great visibility, and good guns. Life is good above 250 mph. Try to roll a spitfire in a high speed dive and you'll think the mustang is fantastic. The 50 cals have good ballistics and a high firing rate. If you learn where the convergence is, will light up your opponent instantly.