I have the same cards. Mining at 30.5 mh/s per card :/ I flashed the sapphire pulse hynix 8gb rom to them and worked wonders. Used this http://1stminingrig.com/best-bios-rom-for-sapphire-pulse-rx-580-8gb-oc-hynix-memory-30-mhs/
please only do this at your own risk. Just because it worked for me does not mean it will work for you. Make sure you backup your original bios!
I underclocked core to 1130, memory to 2175, and undervolted by 150mv. That gave me 30.5 mh.s for each card.