The density of the air at sea level is the same at all sea levels (on all maps). Above sea level the density falls off appropriately.
Interestingly enough, though, I asked an interesting question once. I asked if it would be possible to make a map with super dense air, as in make the terrain at -5000 feet, or something, and would it make for challenging gameplay. The answer was they do not model it below sea level, sea level pressure extends forever downward.
I admit my request was rather gimicky, but I was more curious for novelty reasons. The answer was nice to know.
P.S. The split flaps on the typh are really not very good. They also seem to cause a lot of instability (probably because of the massive torque).