This car was always coming -- from the second Audi put a pencil to the new A6 four years ago. Even though the A6’s V8 engine came out last year in the S8 (and the Bentley Continental GT), the A6’s engine bay was always meant to accommodate it. Wait until the A6 is nicely settled in, then spring the faster, stronger S6 on the waiting world to satisfy the people who don’t think the standard fare is quick enough.
The only trouble is the standard fare is already plenty quick enough, especially (oddly) in diesel form. And it's cheaper, too.
One other minor issue is that too many people expect the S6 to be an M5/CLS63 AMG rival. It isn’t -- that job will wait for the RS6, with another 100hp. Alas nobody quite knows when that will be.