There was a LO Agility MixGross discussed sometime ago in the ""creative" movesets/EV spreads" thread a while ago:
http://www.smogon.com/forums/showpost.php?p=850578&postcount=804
One of the better I've seen around, coupled with damage calcs for most of its common switch-ins, 2HKOing all of them.
The issue at the time was, to either go with the definite OHKO on Cradily with the current spread, or swap 4 EVs from Atk into SpA to get a reliable 2HKO on Cune with GK, while missing on the 100% chance of OHKOing Cradily with MM. Just pick accordingly to what's used the most atm and/or what you "fear" the most.
Like Lord_Sunday mentioned, you can post a Peer edit in the Analysis Workshop section concerning a Mixed Metagross set if you want.
I think if there's a full Special Attacking set listed there, we might as well have this one (or another Mixed set that works good) up in it. After all, Icy Wind is mentioned in the Options section for an unexistant MixGross set. I can't run tests with it though, so I'll leave that to someone better qualified than me.