as far as I know there isn't one out yet specifically for the M6 in the U.S. or North America. That being said, you could still have someone fab one for you, there are quite a few performance exhuast systems from Borla, brullen, and magnaflow that are generic, and can be tailored to your vehicle. However, these might not perform as well as one built for the sole purpose of the M6. Its not hard to replace the actual mufflers, getting a full cat back system on the other hard would be more work. I would wait for duratec performance, or one of the JDM companies to do a custom job for the M6, more bang for the buck.
i was thinking the same thing... my 6 has a nice sound to it, but i would like to have it a lil deeper and try to unlock sum more HP if i could. that way i'm ahead of the game and am waiting for the CAI.