The Pokémon Company has just released a video showing Mega Metagross ingame, as well as showing some the Gyms from Hoenn as well as showcasing Super Secret Bases. Although not mentioned in the video, the Omega Ruby/ Alpha Sapphire official site has posted that Mega Metagross's ability is Tough Claws and that it gets boosts in Attack, Defense, Sp. Atk, Sp. Def, and Speed. The site also makes a point to mention that the speed stat is "increased greatly). Steven has been confirmed to have a Mega Metagross on his team. Mega Diancie has also been updated in the site as to having an increased Attack and Special Attack.
In online play (to which I'm referring to Battle Spot), and VGC play has seen Metagross kind of fall of the map after Steel's resistances to Dark and Ghost were removed as of X and Y. Due to this, the once feared robot behemoth slowly started to fall into obscurity. As seen in the photo below, as of today Metagross isn't a predominantly featured pokémon on teams even though there are several pokémon that have still held on to their popularity since Black and White. Garchomp, Gengar, Rotom-W, Tyranitar, and Cresselia were all team staples during 5th gen.