I understand the S6 and RS6 will be V6's, which is part of the reason I bought a C7 S6 Avant.
No doubt the new engine will be at least as powerful as the C7, but the way a V8 and V6 deliver power is entirely different.
My S6 was bought as a daily driver and I like the understated "lazy" power delivery of a V8 in daily driving aspects of life, there's a reason most Rolls and Bentley's and luxury cars in general used V8's, it was easy and effortless.
V6's need to be hustled. Which can be very enjoyable, but not in a limo daily type car.
This is not about performance, it's about the character and the way the engine delivers it's power. My weekend car is a Mercedes Sl400 (3.0tt V6) and about a year into ownership I realised I should have bought the Sl500 version instead.
I love our S6, the reality is it's probably the best all round daily I've ever owned, however I think the badge is a misnomer. It would be more appropriate if it were badged an A6 4.0 TFSi. It's just not a sports car like most of the other S models are (S8 excluded!). I come from a Mercedes background and the I bought an S6 as Mercedes no longer sell an E500, which is what I think the S6's closest competition would be if it still existed.
So the new models being V6's just rule them out for me. I bought our 2017 S6 with a 5 year warranty and I intend keeping it a long way beyond that.