If u got 4mhs per card try to connect monitor cable in each gpu, let it 5 secs then switch on next till u do all 6 of them 
that worked for me when i got 4mhs.
As i noticed 512kb size of ROM file is when u save ur bios with atiflash , and 256 is with gpu z. ALWAYS USE AtiFlash if you dont want to brick ur card.
rx570 4gb hynix&elpida i copied straps from 1500 to 2000. running 27.3 on almost every cards. Some cards gives me alot of errors so i reduce mclock and i get around 26.5 stable.
It depends on the drivers, u need to find the best one for you, i used 17.7.2 on 1 rig with 6x rx570 4gb on other rig with 5x rx 570 17.7.2 caused many problems so i used mix of 17.5.1 and 16.11.5, on 3rd rig 4x rx570 17.7.1 and so on Right now i am on blockchain beta drivers with AB beta 15 and im fine with them.
that worked for me when i got 4mhs.As i noticed 512kb size of ROM file is when u save ur bios with atiflash , and 256 is with gpu z. ALWAYS USE AtiFlash if you dont want to brick ur card.
rx570 4gb hynix&elpida i copied straps from 1500 to 2000. running 27.3 on almost every cards. Some cards gives me alot of errors so i reduce mclock and i get around 26.5 stable.
It depends on the drivers, u need to find the best one for you, i used 17.7.2 on 1 rig with 6x rx570 4gb on other rig with 5x rx 570 17.7.2 caused many problems so i used mix of 17.5.1 and 16.11.5, on 3rd rig 4x rx570 17.7.1 and so on
Right now i am on blockchain beta drivers with AB beta 15 and im fine with them.