I am not sure if I can paste hyperlinks here, so I will just tell you to search for Anorak tech website and "Sapphire RX 570 Nitro+ OC 4GB Elpida"
The manufacturing company is different but the memory and gpu are the same and that's more important.
Search for a post from:
USER: Deecept Aug '17
Try this one
rx570_1150-2100_rev2.rom484 (256 KB)
you bios flash one card and see if it's better.
Don't forget to overclock the memory (with MSI Afterburner for ex.): GPU clock 1150 - memory 2100 mhz
PS: as I wrote always have a backup of original ROM.
So I actually tried this yesterday and was getting about 29Mh/s without overclocking on the one card! Thought that was great, but then my screen goes blank after about 6 minutes and I have to do a hard shutdown. Not sure what is causing that. I had only the 1 card plugged in and it never got above 70C