675MX is in the high end 2012 27". That chip is Kepler based, unlike 675M. Confusing for sure. This site is helpful, but has yet to add benchmark numbers for the 680MX though their notes make it sound like it'll be about 1.5x as fast as the 675MX.
http://www.notebookcheck.net/NVIDIA-GeForce-GTX-680MX.83519.0.html
http://www.notebookcheck.net/NVIDIA-GeForce-GTX-675MX.82580.0.html