Upon Mega-Evolving, it gains the ability Tough Claws and remains Steel/Psychic type. Its Speed is greatly increased, but also has small increases in each of its other stats too. - Source
Tough Claws boosts the power of contact moves by 33%.
Metagross as a maximum of 262 Speed, and it says that Mega Metagross's Speed is greatly increased. I think that the Speed will get to 320-350 and that is pretty good!
Also with the new ability to him, making Bullet Punch and another contact moves to deal more damage.
We don't know the stats of the Mega Metagross but Jolly will be the best nature that I see now, they only say the speed is greatly increased making Mega Metagross na outspeeder with a great base stats on Attack with that ability. This can almost be compared to a Mega Aerodactyl.
So yeah, use a Jolly one with 252 EVs in Attack and Speed, but since there are no news about the stats, we need to wait.
Hope this helps.