Thank for your help
Do you have Red Devil or Red Dragon?
After BIOS mod, my RX 580 8GB Red Dragon with Hynix memory does 29.5MH/s (dual mining).
Memory clock 2100, core clock 1150, power limit -20, core and memory voltage both at 850mV.
I'm using Anorak's powersave bios with 1750 straps copied to 2000, see
https://anorak.tech/t/powercolor-rx-580-reddragon/5278 .