Can anyone help me get higher hash rates?

I bought used Sapphire RX470 4GB mining edition cards and I don't have the original bios to mod it. With current bios and others that I have tried, the cards give cca 26Mh/s @ 1150/1850.

If I raise the mem clock above 1850 I get errors instantly. No matter the BIOS.

The cards have Samsung memory. I'm using win 10. Cards are overclocked with claymore miner.