I made the mistake when I first got my Roadstar and my wife accidentally touched her boots on the hot exhaust, leaving bad burned rubber stains, of trying everything except what ended up working until it was too late. In the process I scratched (lightly, but noticeable) the chrome trying a variety of cleaners, abrasives, foil (don't remember where I read that one!) and fine steel wool. After my brain kicked in and I tried lacquer thinner (when pipes were cool, of course) that it takes it right off. I had already purchased and installed aftermarket pipes and now my stock pipes are stain-free, but lightly scratched. Live and learn. Runs much better with after market pipes anyway and it's LOUD too! (sorry for the lengthy description)
Summary, lacquer thinner removes burned rubber/stains on chrome exhaust.