This does sound low for a RX570 on Monero, I get almost 500 h/s on an R9 270x 2GB mining Cryptonote, and 750 h/s on an RX 470 4GB.
But what I found is that the R9 270x runs much better on Claymore v9.7 than it does on v11.2, so try using earlier version of Claymore and see if that helps.
I have the same card (R9 270X, but with 4GB) and haven't found v9.7 to be quicker than v11.2, the opposite in fact. So there's probably something else influencing the performance. I get 470H/s with v11.2.
Definately something strange going on.
I too have an R9 270x 4GB (it's on my main 'working' PC and not used to mine) and a quick test (overclocked with Afterburner) shows that v9.7 gave 487 h/s and v11.2 gave 463 h/s - did the test twice with same results, go figure ?