What are your OC settings in smOS? For an RX 580, I'd give CORE 1150, MEM 2150, CORE UV 800 a shot.
CORE 1150, MEM 2150, CORE UV 855 (I have problem with better undervolting)
I try sgminer-brian112358-x16r-v0.4.0 for now and i get better hash with -x and -w parameters, i have not the final numbers for now, but i got 31 Mhs for 6 GPU for now.
Have you stock rom, or modded?