My take on this is, if someone like Joe has the means to actually see the difference, I'll go with that.
I don't get a chance to put V Power in often as possible there isn't one that close. However, we have a Tesco near us so use Momentum 99. (Even cheaper with fuelsave point.)
This last week I had a gas training course to attend just outside of Birmingham, a trip of just over 200 miles from me. The car had no bars left on the gauge so living dangerously. Haha. I put in £40, drove to Birmingham and back then drove to and from work on Friday and Saturday and still have 2 bars left on the gauge.
I'll never get exact MPG results from that, really not scientific enough, but I was chuffed with that.
Higher octane fuel will always be better, whether you can see the results or not. You'll likely notice the difference more when you change your spark plugs.
As someone said, this topic has been done to death but you'll always have your own preference.