The stock front flaps should work and the rear gli should work, same valance
Clearly I wasn't clear enough, the rear GLI guards do not fit on the cup car, period, is that clear enough?
The rear valance may look similar but the "line" where it bumps out on it is not at the same height. The flaps don't line up with the body lines or holes, so they would need to be modified or they would have to hang down about an inch below where they should end to line up with the body lines. I don't understand why someone would post something "should work" when they haven't tested it and when someone has conclusively done so said it won't work.
The stock front flaps will probably not work, because the rocker covers aren't the same on the stock Jetta as on the Cup Car. Though I have not tried this I am 99% sure they won't fit. If someone else wants to waste their money trying be my guest.
The body kit for the cup car is unique. It is made by autotest. I don't know if they also made the GLI's bumper covers but that might explain why they may look similar but are not the same.