Whilst I have no recent experience of premium fuels, this ongoing discussion reminds me of an issue I had many years ago, albeit relating to diesel fuel, in a Landrover Freelander. The vehicle was only a few months old, when I noticed 'odd' engine noises. I reported the issue to my dealer and his first question, was to ask what fuel was I using. I reported 'supermarket fuel' to which he responded, try 'branded' fuel, which I cynically thought, what the heck no difference!
However i did change over to 'branded fuel', and the 'odd' noises disappeared, not only that but the MPG improved as well, which covered the extra costs. So rightly or wrongly I've never used 'supermarket' fuel since.
However just to counteract that story, I've also heard that wherever you buy your fuel from, ie 'supermarket' or 'branded', the delivery tanker drivers, all fill up from the same tanks, at the fuel distribution centre, so is it all in the mind?