There are wonderful turn-based air war board games out there dating back to the Avalon Hill days, covering all periods (WW1, WW2 and Modern). GDW has a wonderful game, if it can be found, called Air Combat (covering Modern Jets). The technology has changed but the tactics haven't. High altitude, by itself, isn't a bonus. There's a counter to everything in air battles, based on plane and pilot, and in modern age, missle quality. The goal is to get on the opponent six and has been for almost 100 years. This has been negated a bit due to radar missle and the ability to launch from any angle, but heat seakers are still based on oppoent six (or close to).
High altitude means energy, which means more options. Speed in a unmanuverable plane, like a P-40, meant high speed passes at the enemy, extend away, climb and turn, do it again. Against a Zero, this tactic often didn't work and why the Zero was dominant early war. The Zero could out climb and out-manuver the P-40, so it could negate the P-40 dive ability leaving the P-40 pilot with two choices: 1) fight a manuver battle against a more manuverable plane or 2) dive and run away. Of course, a good pilot in a P-40 could still beat a Zero, but it wasn't based strictly on altitude.
IMO, there should be a few things tweaked with the air combat:
1) Altitude bonus, by itself, should be removed...altitude is pretty much already included in plane air to air ability and pilot quality...all things being taking into affect, altitude should only affect the ability to "bounce" (surprise) the enemy and "bounce" should affect the first round of combat only (assuming the game does round by round combat).
2) There should be a true "bounce" check. Though lower altitude planes might not be able to exploit a bounce, it was possible IF the planes could climb fast enough and the altitude difference wasn't extreme. Bouncing comes down to how fast you can surprise the enemy. The check should go for both sides, with a bonus to the higher altitude planes. Pilot quality, alititude difference, climb ability, radar (if present) should come into play. Even if the higher altitude planes get a "bounce", if there's a 20,000 feet difference, the higher altitude planes might not be able to exploit it because it takes time to drop 20,000 feet and the defenders could see them coming and manuver accordingly...If someone wants to fly at 30,000+ feet, fine, but it will significantly reduce the ability to bounce someone at 10,000 feet.
The two changes above, are probably within the scope of changes to this game that are possible. There's another option that may not be doable.
3) There should be a "spot" check, with altitude, weather, pilot quantity, etc., coming into play. If there are multiple squadrons at multiple levels, who spots whom and at what level would have major affect. High level cap spots high level sweep but misses low level bombers/escorts. Mid-level cap bounces low level bombers/escorts, but gets bounced by high level sweep or misses high level bombers (which is what happened at Midway...fighters went low to attack torp bombers and missed high level dive bombers)...
This would open the air war up a bit from the current "fly as high as you can" approach...Fly cap at high altitude to get a bounce and miss either the bounce or the low level enemy altogether...fly cap at mid-level to bounce low level bombing raids and possibly get bounced from high level sweeps. Fly sweeps at high leve (30,000+)l and miss the cap entirely or the bounce.
To bring up another point about air combat. Japanese started the game with superior fighters and superior pilot quality (with combat experience from the war in China). This isn't really reflected in scenario #1. The zeroes were their top plane, and the zero squadrons (at least land based at Formosa) start with a rookie average quality. Hmmmm.....scenario #2 is the other way, with almost ALLl starting squadrons averaging 75-85 pilot quantlity. Not sure if that's correct either, but it's closer to the mark.