I have 8GB RX 570s too but I'm only getting ~980 with my Samsung ones and ~810 with my Hynix ones. What strap settings do you use? Could you share them here please :-)
mostly Samsung, except the two lagging behind - those are Hynix. All modded with straps from 1.6.4 PBE (there's a link few pages back)
all Nitra+ 570 8GB