As far as communities that do arenas and RBG's, theres alot of top players on Shadowmoon. As much as I hate to admit it, XOXO is prolly your best bet in ally side. Horde side is pretty scattered. But theres still a ton of very talented players on horde.
Not to mention, RBG groups can be multi server. Just find talented people, get their battle tags and invite them for RBG's.