Make sure you get your cards with Samsung memory as with a bios mod you can make them go up to 31 Mhash without problems, should do 27-28 Mhash without bios mod though.
I am talking about the best brand in ATI cards,especially for mining, SAPPHIRE NITRO+ is the only kind of card you need for RX 580, just make sure as I said them to have Samsung memory to get the most out of your cards for mining.
I have Hynix and I dont have any issue getting 31.5-31.8. XFX brand
Same here, all my xfx cards with hynix mems are doing over 29+. I thought they were sh*t at first.. but with some advice from some members on here and some tweaking. They are doin well over 29(windows) and power consumption is decent.
Also when you buy gpus how can you ever tell what kind of memory are in there? Esp when you get them in bulks. There is no way to determine what mems are in each gpu.
If you know how to, definitely share this information with us.
