I buy the same Gygabyte RX590 gaming Rev2 at black friday and it's just horrible... stock i have these speed... i didn't understand how you can have 27.5 on ETC, i have 10 on Clay and 15 on phoenix...
You have to switch on compute mode in drivers. It will give you 25-26 mh/s even without BIOS mod. Then try to use strap 1 and rxboost options in Claymore miner.