I ordered the mounts from the factory, then bought and installed the Sparco belts with Bernardo when the car arrived. The pic posted by Pro_GT_Driver accurately shows the sub-belt mounts. My car came with spacers and bars on those studs - that was the only difference between my car and the pic.
After I installed with Bernardo, I had to take to dealer to reset/re-calibrate the airbag weight sensor. Apparently that's a must when you remove/replace the seat bottom. Dealer also had to disable the indicator that tells the car whether the seatbelt is fastened so that it can start up.
Bottom line for me is that the 6pt belts have a very high "cool" factor for about 3 weeks. After that, they're annoying. They're annoying if you or passenger is dressed up. They're annoying if you have a passenger that doesn't know how to use them (imagine hooking up the sub-belt for your mother in law). They also flop around a bit and are prone to damaging any bare CF in the interior (imagine them laying in the door jamb and your passenger tries to shut the door), which is annoying.
I found that I was constantly managing the belts as much as I was enjoying the car. Today, mine are still installed but tucked behind seat. I'm using the 3pt system and have not looked back. Am I glad they're there? I guess so, but I am not as thrilled about them as I thought I'd be. Have I "tracked" the car or done anything else to justify their presence? No. Will I in the future? Maybe, but the belts wouldn't de a deal-maker or deal-breaker in that regard.
My advice is to order the option but NOT install/use the 6pt harness unless: a) you are competing with the car, b) you have had experience driving a streetcar with similar harnesses and know the deal, or c) are the kind of person that really enjoys doing the "gonzo" version of everything.
Hope this is helpful.
-Stuart