Im using 2xRX580 and 1xRX570 in one RIG with the new driver. Get from RX580 about 30,7Mh/s constantly, the RX570 Nitro+ delivers 29,6Mh/s - with a patched BIOS.
The undervolt process is done by the OverDriveNTool-which works good - so the whole RIG with these 3 RX and a constant rate of 90,8 Mh/s pulls 400 watts at the wall.
Below 850mV the RX5xx will get unstable at MEM 2100 and GPU 1150 Mhz. I think more optimization is not possible with this technology. Let me know if I'm wrong.