Gigabyte R9 280X OC WindForce comes factory overclocked for the GPU, 1100MHz compared to reference 1000MHz. Memory works at reference 1500MHz (6000MHz GDDR5 effectively).
We could push the memory up to 1600MHz (effectively 6600MHz) and for the GPU we hit 1155MHz. The overclock was full stable in all tested games. It could be simply that we were not lucky and we got the GPU which was not willing to work on higher frequencies. The overclocking yielded a 4% to 7% higher result in games.
We overclocked on stock voltage and fans set at default. This card does not support voltage tuning. You can use Gigabyte OC Guru II for fan regulation. However for overclocking we recommend AMD OverDrive or MSI Afterburner tool.