i have read 11mm is built into the xray wheels. i was told to buy +6mm hubs and then i was told to buy +11mm hubs. the king headz +11mm hubs are what i'm running at the moment and the work well. they are also listed for the xray xt8 specifically!
those should work great with the proper adapters. i like the grp cubics, they are awesome in most conditions and they have two ribs that really help out in corners.