Does Raising the Memory Clocks on a GPU do anything cause this may sound a bit absurd but I pushed my GPU this is my Card Overclocked and Its Stable I ran Multiple GPU Stress Tests and I didn't see no Artifacts at all. if thats hard to believe.
Raising the memory speed will help in certain instances as it raised your memory bandwidth. It comes down to your core to actually utilize it as well so an oc there will make a bigger difference. On older gddr3 cards the memory oc made a big difference.
I'd up the core as much as you can with the memory at stock. Once you reach a stable core, then raise the memory until fail, then back off a little on both for longevity.