P=51D
P-47 D25/D40
The P-51D's Merlin engine increases it's power from the deck to 12K. After that your turbo charger starts switching gears and you get your power back at 16-17K, then your good up to 26K. I personally suggest you fly your P-51 either at 12K or 17K, because under 10K your going to be to low and between 12 and 17 your engine does not work well.
The P-47/D-40 on the other hand just handles better and better with altitude. I do not really know that airplane well so I can not tell you which altitude would be the best, from what I seen they are very good at 20K.
Try not to fly either one of thous airplanes under 10K when your outnumbered.
Let me know if you have any more questions.