I spent some time today checking out the Brabus about to be delivered in Hartford. The exhaust appears to be the same stamped-steel unit that's on all of the other cars, except for the location of the tips, which are covered with some chrome cans. There's nothing to indicate that it flows any differently. John told me that it sounded the same as the other cars to him.
The Brabus fuel door was just a stick-on trim addition. For the price, the whole package left me kind of cold.