I bought a Sapphire Nitro + RX 570 and a MSI Gaming X RX 570 and have the same problem with both cards: I'm stuck to 24Mh/s
I tested different bios (modded from the original bios) using the 1500 strap, I then tried many straps either from my bios or from the internets.
Some time I can get up to 25Mh/s but never more.
That's weird as I run a RX 470 with no problem to 29+ MH/s as well as two RX 480.
Both RX 570 use Hynix memory. But well, it should do better than that, isn't it? I've seen in posts here that other people have the same issue with the MSI but having the same issue with the Sapphire is strange.
I'm mining using sgminer on ubuntu. Also tried claymore with similar success. Tried different ram OC, they handle it with no crash but even at 2100 - hashrate is the same.