I did a few tests over several tanks of fuel. Very different to get solid results as there are a whole host of variables, however i saw some interesting trends.
Firstly, the car idles and runs smoother in low revs on any performance brand (Shell / Momentum). I've heard that the car can take several tanks to adjust to which ever fuel you've changed to, so perhaps this went against the 95ron when i switched to that.
Secondly, general pickup and sense of power is greater with the super unleaded. Very open to mind games though, so perhaps don't take that as gospel.
Thirdly, i track all fuel expensed on my phone. I happen to do very regular runs, so my variation on driving styles remains fairly constant. I found premium fuels gave me 30-40miles more per tank. I get around 310 with 95 ron. Around 340-350 Tesco Momentum.
Say thats a gain of 30 miles. That's circa 10% better mpg.
Standard Fuel costs* : £1.30 per litre.
Premium Unleaded costs: £1.38 per litre
That's an increase in price of roughly 6%.
10% - 6% = 4%To sum it up, you gain an extra 4% economy (at least i do), and you also get a car which feels faster, smoother, and probably makes the car feel warm and fuzzy inside.
Hope this helps

*taken from
http://www.petrolprices.com/