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.
I can confirm this. Those are good speeds for an RX 570. No reason to try and push it any further. You will use more power and possibly damager your card. It is best to find the sweet spot between power, hash rate and not reducing the life of the card too much.