Whether I won or had to disengage I'm watching them descend on me with little understanding how to combat. I know I need more time in the TA, but how do you handle this? The barrel roll defense is something I've tried with limited success (I'm sure it's my execution.)
Ultimately you want to neutralize his altitude advantage, and even with aircraft that can outclimb you that's not impossible. If I'm alone and dealing with a high con I want to maintain a bit of horizontal separation. It's a LOT easier to neutralize his advantage if he's not able to stay right on top of you, so what I'll do is:
1) Once I ID him, I'll turn away and show him my tail and start a shallow climb, maintaining as much E as I can. This buys me a little time to neutralize some of his alt advantage since he has to close the gap horizontally. I keep an eye on him and watch. If he continues on his way I may go into Auto Climb (Alt-X) to grab as much altitude as I can before resuming course. If he turns in and starts to follow I watch his range...
2) ...and once I see him close to within about 2000 yards I'll go shallow nose down to pick up a bit of maneuvering speed. My eyes are still on the bandit.
3) At around 1500 yards I'll enter a slight nose-low turn into him. This will start to eat up the distance between us as he goes to cut the corner, but he'll also have to commit more to keep guns on me. This sets up...
4) ...my move at about 600-800 yards, extreme gun range. I break hard into and back underneath him in a Low Yo-Yo. If my positioning and timing at the break are right he won't be able to follow.
5) I pull through the bottom and go into a zoom to grab back as much altitude as I can while I IMMEDIATELY pick up where the bandit went. If I see he recovered and is grabbing altitude again I'll keep repeating this until either we've neutralized E states, he loses interest, help comes, I can get away, etc.
What happens a LOT however, (especially against P-51s) is that the bandit over-committed and is now well below me, blowing a ton of E trying to pull out. In which case I cackle gleefully, kick the rudder over and drop on him.

The real key to this is horizontal separation, since that gives you the room you need to recover your own E between each pass and sucker him into over-committing on his attacks.