I believe, the psi-efficiency is already depending on the distance to the object. Some other idea: Make the cost 80% TU. (It takes time to concentrate, your busy, yadda, blah) I mean, especially in the early game, when you encounter Sectoids attacking you with psi, it sometimes seems, as they have far to much fun and on the other side, in the late game, it really is rather easy, once you have 2-3 rather strong psi-soldiers with 75 TUs+. That way, you can control the entire crew of one of the smaller ufos with only 3 soldiers...