The same situation, but i cant even reach 860 h/s.
Recently i bought 5 Asus Dual OC RX 580 8gb with hynix memory. I flashed it with uber mix straps, in stock it showed 570 h/s, after flashing - 772 h/s. Changing cclock and mclock hash only a slight effect. With cclock 1100 it shows 760 h/s and with 1360 - 772 h/s. Exactly the same situation with mclock. MSI Afterburner and wattman show that clocks change correctly. I also tried to change -dmem and -h, -dmem 0 decreases hashrate to 550 h/s, changing -h has no effect. MB - BIOSTAR TB250-BTC.
I also have another 1 rig that runs 3 Asus Dual OC rx 580 8gb with samsung memory and 2 asus dual oc rx 580 4gb with elpida memory.
I was able to get stable 1020 hashes with 8gb version (cclock 1330, mclock 2000) with samsung memory and 925 hashes with 4gb version with elpida memory. MB - AsRock Z170 Extreme 3.
I tried differrent miners with different settings - no effect, the hashrate even lower.
Mb smb can help me with an advice?
I've read so much advice in the last few days and it just seems like the Hynix memory is inferior to the Samsung. If you also have an inferior make of card (I've now realised that the Gigabyte RX580's I have are not the best) AND Hynix memory, you're pretty much stuffed.