I don't think there'll be enough MegaPokemon to justify making a new tier. Plus they're all at different levels, I mean MegaMawile sounds good, but there's no way it'll match up to MegaMewtwo. I think they'll just be banned if they're too good. And so far I'm betting on MegaLucario and MegaBlaziken getting banned really fast.
It doesn't give a 100% attack power boost. It just that Mawile gets the ability Huge Power in that form which doubles its attack in battle. The MegaMons do get stat boosts though.