I think Sableye belong exactly where it is in OU. Mega Sableye may be somewhat different, but it's still just as much of a monster, and the addition of a Mega Evo makes it more unpredictable.
You can still run WoW, Calm Mind, and Recover to set up or cripple other mons before Mega Evolving and make opponents hesitant to use status moves on it or hazards. In XY it did belong in UU, and now I feel it does belong in OU
As far as mega stones being banned, that's only really been done in regards to OU/Ubers before.