You're doing good Sun.
The other factor is VOLTAGE for the processor. YOU CAN increase the voltage to help the processor (GPU) run at a higher speed versus stock. The voltage helps stabilize the gpu so it has enough UMPH (HorsePower)to run faster than normal.
HOwever, the best place to start is simply by increasing the SHADER SPEED gradually and let it run awhile each time to make sure that it is stable and not re-starting itself due to instability. THAT is when you can try to adjust the VOLTAGE of the GPU, to make it more stable.
Hope this helps,
micro...