There's a video on YouTube showing an S8 with before and after dyno tests and the carbon clean showed an improvement.
https://youtu.be/7ohS1MGA26Q
From my research, the symptoms of an engine clogged with carbon are rough running when the engine is cold and reduced power. I was experiencing the first, but I have to confess I've not driven carbon-free S8s enough to know if I also suffered the second. I've always felt the engine had more to give though.
I didn't do a dyno test before it went in to have the clean. Even if I did, it would have been inconclusive because of the additional work being done to my engine. A colleague is an engineer and the very first time he saw my car, he told me there was something up with the intake. He listened to it and told me it sounds like it was sucking when it should have been blowing. It could be that the problems I've described were down to the broken intake flaps.