Look in Sec. 7.E, P.114 of the 2012 rulebook.
"... The exhaust system, looking at the end (down its centerline) shall be unmodified, i.e. the exit diameter of the canister (muffler) cannot be enlarged. This comparison will be made when the bike is assembled as ready to run. Any performance modifications must be out of view..."
If you're planning on running in a Production class you'd be wise to carry a couple of pieces of the factory's literature, handbooks, and shop manual. they may well be requested by the inspecting official, and not having them could be "grounds" for tossing you out of impound. Remember that the pre-racing inspection is mostly for safety items, not class confirmation. Sometimes you'll have a safety inspector point out something that won't make it and you might as well go change it now. But you will likely be asked about it in Impound.