I bought 5 each of Gigabyte Gaming 580 8gb, Gigabyte Aorus 580 8gb, and Msi 580 Armor 8gb. The Armor cards are absolute junk and I'm sending them back. Modded one of each card and got both Giga versions up to 29mh/s stable, Msi would never go over 26mh/s but mostly stays at 24mh/s and the mod and/or changing core/clocks only makes things worse. TBH I'm thinking about sending all the cards back and waiting on Vega. If these cards had done ~30mh/s they would have almost paid for themselves by the time Vega releases. MSI Armors are going back for sure! Total garbage. Cheap construction, no backplates, just nothing good to say about them.
I have similar experience.
Sapphire 570 Nitro + 4GB - elpida - 24Mh, memory does not go even at 1850MHz, Polaris bios does not work
Sapphire 570 Pulse 4GB - hynix - 22MHs - the same case
Sapphire 580 8GB Pulse - Hynix - Memory timing is totally tragic, but it works by polaris editor, and strap 1750 and runs at 2150 = 27.3MHs - undervolt through afterburner - 175w on the wall on celeron 1151 skylake, Z270
My plan is to go to nVidia - I squeezed out of the cheapest 1060 3 GB stable 23.5MHs at 95-105w per card, 6x 1060 = 590-630 on the wall.
The Asus Z270-A Prime runs on a 7-card configuration. 7x1060 164MHs per RIG.
Soon, the gtx1060 version of the 9000 GHz memory will come out of it, maybe a little more.