First, make sure the power tuning feature ("Power Control Settings" in CCC) is set to +20% (or whtvr the max is). If it doesn't help or if you already set it, download HWmonitor and check a few things during a benchmark by using the logging feature:
-does HWmonitor show the right clock rates?
-do the clocks remain stable throughout the bench?
-does the voltage remain stable? how much does it deviate?