Well there's a difference between out turning and out angling somebody. If you're in a dedicated flat turn fight you're dead against 90% of the fighters in this game. The P-51 has a really awful turn radius.
However, if you have speed and E to play with you can bleed it off and pull some sharp angles for a couple seconds. Just long enough to get kills, sometimes.
Defensively? Don't get too slow. Unless you are absolutely sure nobody is around within 5 minutes' range don't get slow! If you wallow you're a sitting duck. Now, if you slow down to manuver but you stay above a certain speed, you'll be much better off. I'm totally SWAGing it here but I'd say maybe 200mph should be your danger zone, and don't go below 150. You CAN but your performance will suffer. Stick for an angles shot, and if you have to don't be afraid to use your main strength and disengage. Dive away, use your top speed to get out of the bad situation.
The big problem I'd forsee is getting stuck in the mentality that you HAVE to stick it out and fight in a losing situation. Break that mentality and you can disengage at will. Then re-engage once you have separation, E, alt, speed, whatever you were lacking before.
I'm a casual fan of the pony B as well. I like it for the challege of 4 guns. I used to like it because it wouldn't spin out nearly as much as the D in tight turns, but that rarely happens in recent versions of this game. In AH1 it was the main reason to take the razorback jug or pony over the bubbletops.