I taped the WHOLE surface area of both of my sidesills about 1.5 years ago, and I can see no yellowing, BUT the material is starting to show, at various angles, a surface cobweb appearance, which I assume is the heat getting to it. It still looks good to me, and if I wanted to, could remove the stuff, clean of any residue, and do it again.
I know this for sure --- there isn't one gravel nick anywhere on the sidesills, including the bottom --- good as new. Any unprotected sidesill will have nicks on the bottom, front (if not taped), and the bulge (if not taped). Guaranteed. Many don't worry about this road rash, but I do. Next guy that gets this car may appreciate it.
I did the same with my SRT-10. Glad those sills are cooler.