Running rich is a very good strategy by car makers to ensure they don't have cars returned with bung engines
On a stock to lightly modified GT-R, you could easily get away with an SAFC-II, but then they are ~$400 and when you make any major modifications you will probably want to upgrade.
Basically, its overkill, but its more futureproof IMHO. I will be putting one on my (basically) stock GT-R and getting it tuned for local fuel conditions. I expect there to be some small gains in fuel effeciency and by leaning out the A/Fs, some power gains. Quite how large or small, I'm not sure. It might be worth bugging a tuner (Dr Drift, SK, etc) to get some idea of the benefits on a stock car.
Edit: List updated: we are halfway there now folks. Fingers crossed. I meant SAFC not AVC-R
Lucien.