Don't expect much more from it.
I have several 570's ranging from 26,5 to 28,5Mhs.
I haven't seen more then 28.5 in any 570 to date, so yours is totaly normal performance wise.
really? even my old 470 refs are doing better then the numbers you state.
rx570 can go up to 31. Normally however it hashes around 29.5-30.5
I've got a rig of rx570 4gb MSI gaming with Elpida doing mere 29.4. And the only slower 570 I've got is ASUS Expedition hashing at 28.8