I find Momentum just feels like boggo fuel in my 172, idle being just as lumpy as 95 RON. In gear punch though seems alright.
V-Power however, smooth idle and pulls well, key difference for me is how well it pulls between 6000 and 7000 rpm.
I'm doing 100 miles a day now on my commute, and recording MPG properly using the trip and full fill ups, consistent route and letting fuelly.com do the maths. Last tank was a confirmed 36.5mpg (trip computer reckoned 38.5mpg), and I'm no slouch. Last two tanks were V-Power and so is my recently brimmed tank I'm on now. Gonna see what mpg I get on this tank then try a few tanks of Shell FuelSave Unleaded n see the difference in MPG.
I'll report back in a week!