I watched some you tube video where they mentioned you can use plastidip. Haven't tried it myself but it's cheap enough you can try it before dumping $300 to powercoat the mufflers.
I had mine ceramic powder coated which held up just fine.
Edit:
https://www.youtube.com/watch?v=DgiMNQAoBd0
http://www.600rr.net/vb/showthread.php?t=298234