Since everyone else is throwing their two pence into the fray, is it not technically true that Gork and Mork still watch over the crazy Shaman-Sorcerer to stop him being too crazy? Because if it is, and he's still got the gestalt field bending to his beliefs somewhat, it's perhaps plausible that his own belief in their protection is what's actually protecting him from most negative side effects of being a sorcerous heretic, thus reinforcing his faith in their protection. Certainly, he's very powerful as a character, but in my mind, you don't hit Mary Sue levels of characterisation unless their abilities in whatever form (be it powers, social skill/status, intellect, etc.) aren't justified, and the justification I just provided seems quite reasonable for the time being. (Though a threat sufficient to fight evenly with him despite his power would also be appropriate at some point. Really, psykers in general have major advantages over non-psykers, so.)