WTs are not less effective with higher tpa. you fail more ops, yes, but not because of WTs. soooooo you should be seeing "thievery against a target with higher tpa and WTs is less successful than a target with low tpa and WTs."

also, they did away with the "no thieves = no thief losses" a while ago to help prevent farming. even if they don't have any thieves, if you fail you'll still lose thieves.