Not a lot to be gained by going hight at the moment, but how about allowing any plane to play a dive card in addition to a normal manoeuvre card (place it on top to play first, or underneath to play second) allowing the plane with altitude to trade height for speed. In other words, it gets to play 4 cards per turn (in three phases). The cost is one altitude level.
I actually already implemented a simple speed/height system for this game:
I suppose I should have cross-posted here at the Miniatures forum much earlier, but I didn't want to "pollute"... my bad! Anyway, here's the gist of how it works (a full description is in the link above)
Other aspects covered by this mod include a diceless combat system that factors in deflection shooting and the like.