I have Sapphire RX560 2GB 1500MHz with Elpida memory (11267-19-20G). Out of the box I got 292H/s. 
After "1 click config" in Polaris editor, and some minor tweak in OverdriveNTools (GPU clock from 1226 to 1300), I reached 365H/s in Claymore 11.3 using Cryptonight v7
I tried to increase memory clock, but even increase from 1500 to 1510 crashed everything.
As I see some results way above 400H/s, I don't have any option than ask for help.
After "1 click config" in Polaris editor, and some minor tweak in OverdriveNTools (GPU clock from 1226 to 1300), I reached 365H/s in Claymore 11.3 using Cryptonight v7
I tried to increase memory clock, but even increase from 1500 to 1510 crashed everything.
As I see some results way above 400H/s, I don't have any option than ask for help.
Sounds like one of crappy new GPUs...ordinary/good RX560s should have default mem clock 1750MHz, not 1500MHz