Oops, I guess I read the thread a little too quickly. I read it as "I am experiencing decreased runtime with the dragbrake vs none". The post was maybe a tiny bit misleading, but not unreadable. My bad.
Anyway, any braking should increase runtime slightly as most BL ESCs use regenerative braking.
If you are already nosediving on jumps with no brake, adding drag brake will most likely make it worse. You could try shifting some weight to the rear. Also, if the rear springs are too stiff, they could be throwing the rear end up as it leaves the jump.