Usually I give the enemy P51's a couple more % of fuel over my own plane.
Gives you a slight (hard to catch) chance to out-fly it.
Real pilots make mistakes, DCS AI doesn't.
Hammerheads and high yo-yo's normally do the trick though.
I'd like to see AI stall out one day, like a script that adds a bit more stick-pull in every 100th turn or something.