DingHao2,
20,000ft is well in excess of where a Jabo should climb too, especially for a mere 7mph.
Consider that most of the fighters in AH gain more than 7mph going from 13,000ft to 20,000ft. For example, the Spitfire Mk IX does ~375mph at 13,000ft and about 390mph at 20,000ft. So at 20,000ft doing 387mph you are slower in relation to the enemy fighters than you are at 13,000ft doing 380mph.
The other problem with coming in at 20,000ft (I speak from experience in the Mosquito) is that you reach too high of a speed in the dive.
Coming in at ~10,000ft still puts you above most of the enemy and your trip is much quicker. The dive in from 10,000ft works much better as you won't compress. In my experience, if any enemy fighter has bothered to climb above 10,000 to 15,000ft then he has likely bothered to climb abover 25,000ft. P-51D squads, for example, seem to usually be at ~25,000ft.