Not much is available as far as a full system but Remus has replacement mufflers which are overpriced for the performance gains. Quite a few W8 and 4Motion owners have gone the custom route by replacing the mid-pipe resonator with an X-Pipe or a 2-Way Muffler and the rear mufflers with units from vendors like Magnaflow and Borla.
When the search function comes back, check the exhaust system by member VWGuy4Ever which is considered the master-mold for custom W8 and 4Motion exhaust systems.
Will aka The Swami
Modified by The Swami at 5:17 PM 3-8-2009