GTX 580 is faster than GTX 660. Look at the graph I have posted.
I wrote, "on par". It doesn't mean it's faster, but at the same level. 6% difference is so small, and do you believe that from the GTX 480 to the GTX 580 the performance only increased by 6%, I don't think so.
Conclusion: GTX 660 is faster than GTX 480