This 670 has a factory 104mhz overclock, which is already 11% over a reference model. The overclock puts it right on with a stock GTX 680 performance wise which would make it ~32% stronger than a stock gtx 570. I never saw the need to overclock this card any further.
Remember though, not only is it 30+% stronger, it runs cooler, quieter, and consumes less power.