Ill PM you the pricing on the v10 S6 headers and exhaust. If you wanted a set for the S8, it would likely be something like the price of the S6 headers plus whatever the labor and incidental costs are for removing/reinstalling the engine from your car.
The S6 guys are getting a break right now on the pricing, since my engine is already out.
As far as the exhaust sizing, the entire system was designed and tuned to achieve maximum power over a certain RPM range. If you have properly designed headers with a well designed collector, then you dont have to worry about a loss of torque by pairing it up with a free flowing exhaust. Exhaust velocity is what is key, and our long tube header design works to maximize that.
There is a great writeup on exhaust myth that ‘engines need backpressure to acheive maximum torque’
http://www.mustangevolution.com/forum/f312/t296628/
A dual 2.75 exhaust would have been good for up to about 560 hp. A dual 3.0 will remain efficient approaching 680hp, as part of a properly designed system
http://img.photobucket.com/albums/v413/nemo1ner/Exhaustguide.jpg
What’s interesting, is that the most popular exhaust for the v10 S6 (Millteck) is only dual 2.36", which is already borderline for stock v10 S6 HP. No wonder we dont see any real gains with that exhaust.