Perhaps it's bad form to reply to your own question, but ...
I think there are multiple questions tangled up in this one:
- How do you keep minmaxers using the GURPS (3rd Edition) default magic system from running roughshod over your adventures? (The least interesting to me, since I probably won't use GURPS Magic again.)
- How do you keep adventures interesting when your PCs have telepathy, precognition, teleportation, "Detect Evil", or similar abilities?
- How can you block PC's supernatural powers as the plot demands without making those powers useless? Is that even fair?
- Will players accept an RPG where they have restricted or no access to magic/psi/superpowers, but other beings in the world do? How do you keep the GM from running roughshod over the PCs?
BTW, one non-magical way to defeat telepathy is to keep the muscle ignorant of their leaders; they only get orders from a "man in the shadows". "Triad gangs", in which each member knows only two others, would be an excellent natural defense against telepathy, seeing how it was originally designed to guard against a member talking under torture, or an enemy infiltrating the ranks.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)