I use a mix of Tescos and Shell and see absolutely no difference.
Every Little Helps as they say...
You should be ashamed of yourself!
I am, thats nearly as bad as the joke "i want to name a street Skin Street so I can laugh at the people who live at number 4"
According to google maps there is the following road in the US "Skin Creek Road, Lewisville, OH, United States"
Imagine if your skin creaked!
To be fair I dont think you'd ever be able to feel a difference. Have you ever seen any difference on Bill's rollers between Shell and Momentum?
edit: doh i just realised you were meaning between tesco 99 and vpower, rather than SUL and Regular!
had it been mapped for 102 and you threw 98 in it though, you'd see a lot of pull and distinctly less power.
I think MRC did a European S4 recently and the 102octane fuel gave about +30hp on a similarly tuned (~500hp) S4 motor.
ultra poo map