MSI Afterburner works fine here (same driver version with 680MX in Windows 7). My iMac is not available to me right now, so I can't be more specific. I would recommend reading a pdf manual for Afterburner, maybe you haven't enabled some setting or something..
Somehow mine don't, but this could be only cuz of "newer" 775M. I've found setting in Nvidia Inspector, but no clue which are good to be edited <_<




Also thinking about giving it to some service so they can apply better thermal compound.