word up..yes the manual states that the grey belts are for stock racing!!!..the grey works great in the front for Mod also...but we only got around 15-20 runs with a grey belt in the rear before it started skipping. (with a 12x2)
The black belts are super efficient when soaked in wd40, as Racer X suggests..almost on par with the grey rear belt for flex and efficiency.
Combined with the R.W pullies, my car feels like it has almost zero friction, although I noted the Diameter of the R.W machined Pullies is slightly smaller, to free up the drivetrain further. I have found that running the grey belt in the front and the R.w pullies required me to add a little bit of tension on the front tensioner to prevent the front belt skipping a little under acceleration.