Click on the link below regarding the relative performance of the GTX 680m. It is basically a review of the mobile GTX 680M.http://www.notebookcheck.net/NVIDIA-GeForce-GTX-680M.72679.0.html
In the mid column you will see a ranking of mobile graphic cards starting with "Class 1" the most powerful. Intermixed with that list are some desktop cards which are in black text. Based on that summarized list the mobile GTX 680M is between a desktop GTX 580 and GTX 570. The desktop GTX 660 Ti is ranked above the GTX 580 in this relative performance list.
Scrolling down the actual review itself there are some benchmarks that you can look at. It starts with synthetic benchmarks, but further down are game benchmarks. Take the benchmarks with a big grain of salt because in the world of laptops you cannot always test laptops with similar specs other than the graphics card itself. Meaning some laptops may be configured with a dual core i7-3520M while another laptop with a Radeon HD 7970M might have a quad core i7-3740QM CPU. And naturally a GTX 680 (or any other desktop graphics card) would be benchmarked in a desktop.
In Far Cry 3 the GTX 680M performs a little worse than the SLI GTX 670MX which has a 5% performance advantange, but better than the desktop GTX 470 by 10%. The desktop GTX 660 Ti appears to have 16% better performance.
In Assassin's Cree III, the GTX 680M performs better than the SLI GTX 670MX by 9%. But the GTX 660 Ti has 28% better performance. The older desktop GTX 470 is not part of the benchmark results.
Have fun reviewing the benchmarks...