Edit and Update:
I bought 24 new MSI Gaming X 4GB Rx570s, and it took me a while to mod them effectively mod them. Now getting 28mh/s using 70-72 watts.
1150 core clock, 1950 memory clock
You don't have to adjust the voltage voltage, i already set it to 975mv in the bios (but you can low it more in afterburner if you want)
I just want to share my mod for anyone having the same issue, i hope it helps.
Modded Bios + UnderVolting:https://drive.google.com/open?id=0B6mADiEDwrHQWV90bVc1ZkRwM28