There is no more energy content in higher octane fuel. What the rating indicates is a greater resistance to knock, the early combustion of the fuel-air mixture that causes cylinder pressure to spike.
When higher-octane fuel is used the modern engine controllers can take advantage of the elevated knock threshold and dial in more aggressive timing and higher boost pressures to improve performance. ( that is my simplistic understanding)
So the first part of the decision is whether your engine can take advantage of that. Some can and some can't. I am pretty certain, without checking, that according to the manuals, the T4 is recommended to use higher octane fuel to 'get best performance'.
I am surprised that the myth of branded fuel still persists. This link outlines the position and explains about additives.
https://www.allstarcard.co.uk/news-i...ermarket-fuel/
So in summary you may see marginal improvement in mpg but how much depends as ever on your driving style, you may notice it feels more responsive to the throttle and you should notice that the engine 'feels smoother'. It is nothing significant though. I find I can justify the extra costs on the basis of a marginal improvement in mpg (31 to 33 mpg) and the 'feel'.
We use it in both the T5 S60 and the T2 V40 and the dofference is noticeable.
Hope that help but basically try and see if it is worth it for you.
Finally I am surprised that the myth of branded fuel still persists.
https://www.allstarcard.co.uk/news-i...ermarket-fuel/