Have started using Shell V power in my 1.8 HB (two tank fulls now), definitely noticing better mpg, averaged 66mpg first 11 mths from new using supermarket regular and now averaging 76mpg after two tanks on the v power (town driving & regular motorway runs). Definitely more calories in the V power (less bioethanol) but I guess I need to average over another 10 mths or so to compare more accurately (winter driving etc). 10mpg extra seems quite high (have read you might expect 2/3 mpg improvement). If true though that more than cancels out the extra cost and you also get the benefit of the extra additives.
Another consideration re mpg is the size of the wheels, I have the 16in; be interesting to know what that saves you in fuel compared against the bigger wheels.