I do remember read a couple of articles mostly from AA and yes, for newish cars(I think it is safe to say any cars around this century?) even for cars designed for 91, 95 petrol will make it runs longer distance. one number is for 15 litres will runs 3% further runs by AA around 2012. And there is another link I just found from AA:
So 95 and 91 almost cost same running same distance for a 2010 Suzuki Swift. I might be putting 95 on my car.
Interestingly I decided a while ago that since the manually always tell me that I should put 'Japanese regular' petrol in my car, which I think means 91, but a couple of mechanics I talked to saying I should put 95 into my imported cars, I think in that case 91 regular is indeed recommended, but 95 will make cars performance slightly better (around 3% maybe), and they ended up cost about same, so if higher performance/longer distance per tank is your think 95 make sense as well for newish cars which only need 91.