I don't feel like restricting Mentor will do much (even if it might ultimately become necessary).
I honestly don't know how you can think this. Mentor's power is totally unique. Even if restricting Mentor does little to diminish the presence of Gush in the metagame, the idea that restricting Mentor wouldn't do much to change the metagame is, in my opinion, clearly false. It would clearly change things.
I guess I should have clarified my stance (and worded it better). I believe restricting Mentor isn't going to do much to cut down on Gush's dominance. Clearly restricting Mentor will change the metagame, I'm sorry if I made it seem like it wouldn't.
I believe if Mentor is restricted, Gush decks simply change win conditions, but keep their engine. That doesn't change enough, in my opinion, to bring it's power down.