I'm not sure if this is the info you were looking for but S-Cars.org has an article about a SC6 Interceptor, supposedly will only be made available in the United States and Australia. Specs indicate it's a superfast version of the S6 avant, similar to what would be expected in an RS6

Audi hasn't announced it. There isn't even good evidence yet that it is real. Also, once Audi announces it, there'll still be be a six month delay before it comes available, if past cars are an indication.

Couldn't find it. The "News Flash" talks of the S6:
"Yes, the new S6 Avant is coming to the USA! The Euro model Avant is powered by a 4.2 liter 40 valve V8 producing a modest 340 HP. Expect the U.S. model to be similar."<ul><li><a href="http://www.s-cars.org/home.html">http://www.s-cars.org/home.html</a</li></ul>

Sadly, I'm sure this is the last you'll hear of it.<ul><li><a href="http://www.s-cars.org/Features/S6Cinterceptor/s6cinterceptor.html">http://www.s-cars.org/Features/S6Cinterceptor/s6cinterceptor.html</a</li></ul>

I put down a $20K cashier's check to secure one by any means. (S4 lurkers will understand the reference.)