Just tested AA HT in cheat mod. It's still useless in AA. Not because of the power, but the horrible tracking and rotating vehicle. Don't know if it's just bad luck but I tested 10 recons and 10 overwatches. AA HT managed to shoot down 6/10 recon planes and only one overwatch it managed to shoot down all the planes.
For a unit that has piss poor AI power (unless completely out of cover, then it's great), have to pull a handbrake to be able to direct stationary cannon shots (without it constantly rotating), and is a 2 shot (handbrake usually leads to death if you are in range of any AT gun), I don't think that a tiered AA unit should be so piss poor in AA department. Again, it's design is horrible with it's constant need to rotate the front of the vehicle, while it can only fire from the back.
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Did you actually set it in AA mode?