If my assumption is correct the 670m should be 2-3 times faster.Remember 650m is based on kepler.The GTX 680 3 times more cuda cores and 40% higher core clock than GTX580 to be 40% faster.
2-3 times faster? epic math fail? that would be 200-300% faster...
The 670M is ~20% faster
May not seem like much, but it could make the difference between some games stuttering or running smoothly.
Checking game specific tests on Notebookcheck.net would make sense to help decide.