As in, the higher the altitude, the less atmospheric pressure, hence longer trajectory?
HT just said above that this is modeled in to the game . All it means (very basicly) is that the trajectory of the bullet is going to be flatter the higher up you go. In regards to advantages, no. Ballistic diferences at sea level between say, 30mm and .50cal will coincide with the rounds ballistics at 15k on a relative scale.
There is far more to it than just air density being used to equate ballistics of a given round. air density ratio, temperature, baro pressure, speed of sound. All of these parameters will be versus a given altitude.
Do these parameters exist in AH?