I have found improvments in the past but not with the 2015 A8 3.0TDI, it seems to be the same on any diesel..
Also I remember Darkside Development (Tuner) did quite an extensive test and found them pretty much all the same on every measurement apart from price.
https://www.youtube.com/watch?v=C4uO1TPIS30
But if it works for you great, I'm glad it doesn't for me as regular diesel is already too expensive.