Generally, the "throw out the airbrakes" approach to defense is a bit last ditch, but in the P38 you can sometimes catch guys dropping below stall and as soon as they put power back on they snap out of control.
The dive-flaps offered marginal drag when I tested them, it was measurable but barely. Using 1-2 notches of flaps at key points in a fight is critical though, that can make all the difference.
Ack-Ack is dead on about combat trim in the P38. In general it doesn't seem combat trim is a good thing when you get into high, or even low, speed handling problems. It can actually make the situation worse, not better.
I actually have a few AH2 updated pages for my site... I really should get them cleaned up and posted.