"Upon Mega Evolving, Metagross’s Ability changes to Tough Claws, which increases the power of attacks that make physical contact. Its Speed is also increased greatly, and its stats are better balanced than ever! With increases in its Attack, Defense, Sp. Atk, and Sp. Def, this is a Pokémon primed for both offense and defense."

So… Tough Claws for a 33% power boost to physical contact moves.

Increased speed and balanced out stats everywhere else?

…. Oh this thing is gonna be trouble