12 RX 570's would get 340-348 mh/s Eth and 8100 Dcr and use less then half the power, I know your losing 60 Mh/s but the power consumption of 12 570s before the wall is 1500watts vs the 3000 watts of the 390's. You could add 2more 570s to get 405 Mh/s and 9500 MH/s and still be far below the power consumptions of the 390's. Unless you already have all the GPU's or electricity is free this is not worth the investment given mouch new GPU's are much more efficient. If you just want to Mine Eth you could tune the 570s to only consume 100-100 watts per card