I am sure the V power has more than a placebo effect, I mean Shell is a very rich organisation and it is an optimised fuel, even in the 1.3, throttle response is much sharper, revs rise more smoothly and the general feel of the drive is better as you say.
V power is great for a treat in the 1.3 and at 8p a litre dearer in the garages near me, its well worth it.
I find Shell fuelsave to be an extremely good economy fuel too.
I have two shell garages within a mile of me, both quite large affairs.
In terms of loyalty though, just couldnt be bothered with it.