Metagross@Life Orb/Leftovers/Choice Specs
Nature: Modest
252 HP/236 Sp. Attack/12 Def/8 Spe
1. Psychic
2. Flash Cannon
3. Signal Beam/Icy Wind
4. HP Ice/Ground
I have had moderate success. It always takes out the opponent's Physical Wall. Metagross's base Sp. Attack is 95, which isn't THAT bad. On the contrary, it's quite decent. Psychic/Flash Cannon are two powerful STABS, Signal Beam for Cresselia/Celebi/Darks/Psychics, and any Hidden Power of your choice for coverage.
Icy Wind+HP Ground provides great coverage capable of sweeping most teams. HP Electric may also be an option.
If Metagross was allowed in UU this would be decent, but seeing as it won't ever be (hopefully) this is a very bad idea. In OU, a pokemon needs an attacking base stat of at least 100 or extra bulk/speed. This Metagross set doesn't have that luxury. In addition,
Blissey ruins this set, setting up all it wants (and possibly using Flamethrower!) while you do jack shit to it. Also, going up against a special sweeper means getting hit with moves such as Fire Blast, Surf, Earth Power, Thunder(bolt), etc. Metagross' special defense is relatively low, so these will hurt from Pokemon like Heatran, aforementioned Blissey, Shaymin, Vaporeon, Salamence, occasionally Tyranitar, Infernape, Latias(yes it is OU without Soul Dew), Gengar, Zapdos, etc.
A sweeper Pokemon usually needs all the attack/special atk it can get, and then a lot of speed.
Steel-type attacks get horrible type coverage. With 135 base Attack, you can get away with using Steel-type moves on Metagross like the powerful Meteor Mash and prioritized Bullet Punch, but a simple 80-power special move coming from Metagross' 95 special attack is terrible.
If you want a Metagross set like this, read
Metagross' analysis page. It has a mixed set that may appeal to you.