What exactly is the extent of the Psijic's power? They're obviously the most skilled magic users in Tamriel but what exactly can they do? As far as I'm concerned moving their island was a god-like feat but can they do any more?

Theory supported by fact would be much appreciated.

They invented mysticism, so they'd probably be masters of spells from that school.

Mannimarco was originally a Psijic. Or at least studied with them.

Man... I would love to go to Summerset and have them as a prominent (and joinable) part of the story.