the only method i can think of is creating an alternate projectile system where each projectile is then considered an object for which you can keep track of initial height targetting, etc...
of course, if each projectile is an object then it requires some hefty triggering to determine how to target a unit properly, along with keeping track of every projectile as, possibly, but not limited to, a unit (with movement) <-- this in itself will cause slow downs if there are too many projectile-units moving at incredible speeds
uhm, maybe theres another way but thats the one i came up with