Read this link and get the experience of flying the P-51 form somebody who has actually flown one in real life as well as flies the game actively. Miss Strega is active on HL so ask her if you need more information:
http://deltachevron.com/forums/showthread.php?tid=85
Everything and anything about flying the P-51 is on this thread. The mustang doesnt have the acceleration of a FW or 109 and you have to earn the speed and keep her fast. Also most people qouting that she is manueverable only does it above 320kph and never lower and you should engage the combat flaps anyway everytime you go below 280kph in the Mustang.
I use an all 100 INPUT for the mustang ans she flies like a dream specially in WarClouds fighting FW' and 109s. As Erkki's youtube post shows the .50 cals is far from porked and you just need to have SUSTAINED deflection shots and have the proper convergence. I use 300 yards (USAAF recommendation) or 274 meters.