I've used Sainsbury's super (97 ron) many times, alongside V-Power - I get Nectar points with Sainsburys and the loyalty card with Shell. Same as Shuv, I don't really do enough miles to be bothered for the price difference - I fill it up when it gets to the minimum and it costs ~£60-65, regardless of which station I use. The decision is based on which is the closest when I'm driving past.
I can honestly say I've never noticed a difference between either the Shell or the Sainsburys, in all kinds of driving conditions, for better or worse. I don't dispute at all the benefits of using V-Power, however I also think that Shell are very good marketeers.....
When we were up in Scotland for the NC500, we had to fill up with regular octane on one occasion - again, I personally didn't notice any adverse effects, but I wouldn't normally use 95 day-to-day.