I started using Shell V Power and posted some time ago that my fuel consumption was really terrible. I even took the car to VW for a check-up.
I then changed to BP Ultimate and the engine's fuel effeciency improved very slightly. However, the engine felt a bit rougher. Could be my imagination.
I then went back to Shell V Power, and the fuel effeciency of the engine was slightly worse but the engine felt smoother.
Finally, I tried Total Excellium. I got better fuel economy than with Shell V Power and about the same as that with BP Ultimate. However, I felt that the engine was as smooth as when I used Shell V Power.
I am therefore sticking with Total Excellium 97 RON for the present.
While I agree that this is all very subjective and there are a huge number of variables, I can only go by my own meagre experience.