It took quite a bit of googling to find this thread, but I'm glad I did!
First off, I want to say many thanks to the posters in this thread that have done the legwork and research to figure out the characteristics of the stock spring. It appears the dimensions are still the same for my 2012 application.
Unfortunately, it appears I missed the boat by a few weeks (months?). I recently bought a G37 6mt sedan. Only had it for a few days, but can't get over how vague the clutch is. Removed the spring which resulted in much better feel. Given that it's a new car, i'm not to thrilled with leaving it out altogether. So, to remedy the issue and save a few bucks, I started a group buy over at the myg37.com site -
For Sale Group Buy - Clutch Pedal Assist Spring for 6MT - MyG37