XP balance tested 'very well'?
I really enjoyed the game, but I find it difficult to believe that you tested the balance on anyone but yourself. This is one tough bitch. Remember, players who weren't in the design process and don't know the winning strategy aren't going to find it as easy as you do.
I think you could definitely improve it by adding some other upgrades, like perhaps an automatic firing tower--but then that's partly because I hate all the clicking.
To other players:
Your ultimate goal early on should be to set up a cycle, where you snipe them until your clip is empty, and then while you're reloading you call an airstrike. The reload speed, ammo capacity, and aistrike upgrades are all great for this. The damage upgrade is as good as an ammo capacity upgrade, because it reduces the shots per target.
The game says you should hold down the left key, and isn't very specific. Only hold it down for a few seconds--long enough to kill a person--and then release it. THE LONGER YOU HOLD IT THE LESS ACCURATE IT BECOMES.