A buddy of mine always uses oven leaner. He's forever melting either his shoes or rain suit on his pipes. Spray the oven cleaner on a clean soft rag and hold it on the spot for a while, then wipe. Don't try to spray directly on the bike. You'll mess up things with the overspray.. Rinse with cold water afterward.
S100 chrome cleaner. It'll take anything off. I have even put an extra boot guard that I fashioned on my floorboard to avoid running my boot against my heat shields. S-100 chrome cleaner in the red tube does the trick. It's amazing stuff.