Are you using Claymore's Dual miner? What is your intensity set at for Sia? I had to drop my -dcri to 12 to get back to good Eth hashrates. On my Gigabyte Aorus RX 580, I'm at 29MH/s for Eth and 350MH/s for Sia.
Is that with 1500 timings? I can only manage to get 27/600 eth/sc from my gigabyte RX580. They'll only run with the 1750 timings, crashes with the 1500. Dropping SC intensity does not take it any higher.
Yes, I used 1750 timings on the memory, that is the standard way to do memory timings on 480/580s. I tried the Ubermix timings on my 580, but it was completely unstable. I also drop core clock to 1125, and adjust mem clock as high as it will go without getting memory errors in hwinfo. I burn them in for 10-15 mins before committing that to a ROM and flashing. Each card will have different ASIC and memory qualities about them, and you really have to dial in the memory clock so that you don't get memory errors. I don't do anything exotic, but I do drop the gpu voltage 50 75 mV to save watts.