Any semi-decent player would not keep his Mawile in against Sableye unless they have nothing that can take a will o wisp and, in that case, they'd just spam play rough to get rid of Sableye.
Well, actually, Sableye can be pretty good set up fodder if it doesn't have Foul Play and wants to run Knock Off instead, which I know some people do. (Even at +2, Foul Play only does about 39.-47%, so do with that what you will.)
Now anyways, back to my description. At first, I never thought of Mega Mawile as a threat. One hit usually brought him low enough for any priority to take it down, and since Mawile is so slow, basically any priority will outspeed it. I occasionally brought in my Bulk Up Talonflame just to troll it.
However, since then, I've removed the Pokemon that have had priority, and MAN has it been hard to deal with since then. If it gets to +2, it's extremely hard to take down. Its typing makes it pretty hard to find a Pokemon that can do well against it.
Bulk Up Talonflame and Excadrill do a pretty good job of countering this thing, but Excadrill still takes a hefty toll from Sucker Punch.
252+ Atk Huge Power Mega Mawile Sucker Punch vs. 4 HP / 0 Def Excadrill: 249-294 (68.7 - 81.2%) -- guaranteed 2HKO
And at +2 it certainly can't take a hit, but it does help to get an EQ off, and I thought I'd mention it.
Mawile can usually become very predictable. Once you figure out one or two of its moves, you can find a decent check. Notice I said CHECK. Even when you have a suitable Pokemon to deal with it, it will still do a huge amount of damage to anything that decides to switch in.
I'm not exactly sure how Mega Mawile will function in Ubers. It's probably going to be like Mega Lucario, which in my opinion is a pretty big waste of a Pokemon slot. But this thing WITHOUT A DOUBT should not be in OU. Throw it into the dump of over-powered Pokemon in Ubers.
I say
ban.