Yeah come in high, HO or vulch once, and then run. If theres a gaggle of ya then the mustang can be effective. If you make a mistake and an LA gets level with you with close to equal E then he's cooked.
On the other hand theres a few Pony sticks that can give an LA a hard time. The last time I ran into one at TT while flying an LA. He had about 1,000' on me, "which is no big deal", and I tried turning him and slowing him, beating him down lower, and he just slashed and used his energy so effectively I actually got into some trouble and probably would have lost had another plane not jumped in. Not that I'm that great mind you but Ive flown LAs against Mustangs a lot and this guy was really good with his flap work and/or tactics. Normally when you get a Mustang to pull flaps in a turn you got him.
I dont know who he was but he was aggressive and very good. "Most" of the time the hardest thing with a mustang is running them down. An LA can not only out turn it but has a big edge in climb and acceleration.