My collectors have been process coated- similar to jet hot coatings. It keeps the exhaust hot and thus moving faster past the headers. It also allows a quicker cool down when less fuel is being burned. I've removed the stock muffler and put in a straight pipe- it's only marginally louder, but the power increase via but dyno and spoolup time are both improved. Too many folks have complained of droning in the cabin if you remove the resonator for me to attempt this. Going to a larger exhaust can also be time consuming, so these are the quickest, most easy mods to improve the stock system IMHO.