There have been some 'tests' that have only scratched the surface and not been conclusive. To gain an mpg benefit from using V-Power you have to drive the car differently, using lower revs in the gears to enjoy the benefit of better burn at low engine speeds. And, as you write, the real bonus is long-term due to the lack of hydrocarbon deposits in the fuel system.

I don't know anyone who has tested engines over 10,000 miles to compare V-Power with cheaper fuels. A proper test would have to be the type DEKRA use to test tyres, covering a huge mileage in 6 different cars using the two different fuels, continually swapping drivers to get consistent, honest results. As far as I know, Michelin has paid for that but Shell hasn't.