I always put v power in the s8 unless I'm stuck and have to put something else in.
However in day to day driving in the S8 I refuse to believe that anyone can tell the difference between v power and regular.
The reason I run it is for the cleaning properties first then for the better mpg and slight power increase last, coz let's face it who drives an S8 flat out day to day apart from Marcin.
As for BP 5th gear did an interesting side by side comparison of top tier fuels.