Here are a few suggestions of cleaning rubber off of pipes. Well worth a try on leather residue. Good luck
1. Vance &Hines recommends Easy Off Oven Cleaner. Spray on with pipes cold, start bike, get pipes very warm, stop bike, let cool a little then rinse off. (ONLY use the spray on type cleaner. Using one of those Lye based brush on products will melt all of your aluminum and permanently scar your billet). Be very careful not to get it on aluminum it will turn it white .
2.USE STEEL WOOL!!!! All you do is get some warm water/dishwashing detergent solution or WD-40, soak it up in the steel wool, and have at 'er. Oh ya... use fine steel wool, and use moderate pressure. Just make sure you keep it wet and it will NOT scratch. Works great after long and particularly bug infested/road construction tar covered bike trips. If you're worried that it might scratch, test it on some old chrome doo-dad you have lying around or in a hidden spot. You will be pleasantly surprised.
3.Use "Brakekleen" solvent, available at any auto parts store. Just make sure the pipes are cool, and DON'T get this stuff on paint, since it will remove or at least soften any painted surface. Spray the rag, then apply to pipe and rub. Don't spray directly onto bike. This person continually rub her black chaps on the rear pipe and stain the pipe. Brakekleen gets it off every time.
4.Bug and tar remover may work. You can probably get it at any auto shop.
5.Use a penny to scrap off the rubber. It is softer than the chrome so won’t scratch.
