The rig looks fine except don't skimp on power, it's a bad idea in the long run. Not only 3* 730W PSUs is barely enough for 12 RX 580s, but these PSUs are also crap. Buy a couple of high quality 1200W+ units instead. Something like Corsair HX1200, CoolerMaster V1200, or any of the higher-end Seasonics or EVGA's PSUs.
Good point.
I had this thought in the back of my mind, and I think I will do that.