I got from a RX570 nitro 4GB with Hynix and a strap from bughatti(was the best in test of 6 different) in max 28,5 Mh/s out (tuned with 875mV), with a 580 I got 29,5 Mh/s - all stable. The clock max. 2070MHz.
Just to give you a feeling for realistic values.

Update aug 20th: with the new claymore 9.8 and new AMD beta driver blockchain optimized, I get now 29,7Mh/s with the RX 570 and 30,8Mh/s with the 580
https://postimg.org/image/kou9fjcjd/