Haven't played with timings too much, but am using the 1500 strap with nerdralph's tweaks. Running 1150/1750 @ 920mv/900mv, with gpuz reporting an average draw of 74W. Idle draw is around 22W with 900mv vddci (multi-monitor). Gets around 267H/s. Since the machine's running 24/7 anyway, I figure effective mining consumption to be around 52W + vrm/psu losses.
It can do 300H/s at 1250/1750, and around 310H/s at 1350/1750 (memory bottleneck..). Of course, I haven't really tweaked the timings much, so there may be more to be had (stock bios gets around 265H/s at 1250/1750).
You can flash the asus rx570 bios no problem as well. Idle consumption only improves if both monitors are the same resolution

Edit: Looks like I can do 1150@900mv with the rx570 bios (would be stable before but 2nd display would lose sync). GPU-Z reported draw down to 70W now.