sapphire RX570 8gb nitro + memory hynix.
these cards suck
they are not good for mining,
I tried everything, all the timing, in all possible modes, the max to be stable 27.5 mhs
It might actually be your mainboard bottlenecking your gpu, one of my ud3 boards does this to me. That sapphire should go over 30 MH/s no problem.
I'll post a rom for you to try out in a bit, see if that helps.
update:
Sapphire Nitro+ RX570-8GB (Hynix) Mining BIOSWarning untested BIOS!
This BIOS gives me 31.902 MH/s 100 W . Its a beast but will it burn the card too fast?