Rwish: I leased a new 1997 A8 with the 17" wheel/tire option -- put Pirelli PZeros 245 45 17 on the car -- then called Joe Hoppen and he sent an S8 suspension to Northland Audi here in Cincinnati. The components were springs, struts and sway bars -- the cost was about $2500. The installation was in two steps -- one day to do the work and an all wheel alignment -- then in 1,000 miles another all wheel alignment. Obviously far more $ than the factory option -- made a big diff in handling, no real diff in ride quality. Lowered the car about 20mm (.8 inches). Glad I did it -- made me "unhappy" that factory option was only avail in Europe. Audi sport suspensions -- at least based on my experience are so subtle in the difference they make in ride quality -- should be the standard suspension here from AudiUSA. Personal opinion.
This suspension came in a box with the four rings on it. They called it the S8 kit. My assumption is that there is a similar kit that would be called the quattro sport kit for the 2.7T A6 (not the S6 kit which I assume would only be available for the 4.2 A6). Hoppen motorsports http://www.hoppenmotorsport.com/ I have a factory sport package on my 2001 A6 4.2 and I am very pleased with it -- wish there would be an S6 sedan here in USA -- maybe next time.
If you can "swing it" financially go for it -- conversely, for about $3,000 you may get out of your current A6 and get a new one with the 17" wheels and the sport suspension (on a lease).