You know what?? If Kosmic will be the devil's advocate for Iron Head, I will be the Devil's Advocate for Gyro Ball.
Metagross is not Bugatti Veyron, but he is also not a snail. He speed is a bit too high for Gyro Ball to work effectively. But Metagross has one move that can change that, Hammer Arm.
Lowering his speed everytime it hits helps Gyro Ball while at the same time dealing 100 base damage. Not Bad. Both these moves have better accuracy than Meteor Mash and Gyro Ball can surpass Iron Head and Meteor Mash in power, capping at 275 after STAB. The PP is a pain, but does not need all those points after a few Hammer Arms.
275 Base Power coming from 135 base attack is no joke.