From Nvidia's specification, which has a 925MHz core and 1350MHz (5400MHz DDR) memory clock, we pushed the GTX 660 to 1150MHz and 1500MHz (6000MHz DDR) -- an 11% core clock increase.
When testing Crysis 2 at 1920x1200, the overclock allowed us to extract an extra 2fps or 8% more performance.
We recorded a 6% increase in Deus Ex: Human Revolution when comparing our custom overclock to both the Nvidia spec and Gigabyte's factory overclock.
Again, we see a 6% (2fps) increase from our overclock in Max Payne 3. It appears that there is very little reward for overclocking the GTX 650 Ti.