High octane petrol is always preferred in turbo engines.
You will not get more outright performance, it is just more refined to that of lower octane and this way keeps engine cleaner, that is what will give you a bit of performance and therefore better mpg!
I prefer the BP 100 unleaded but Shell V power is satisfactory too!
About supermarket fuel .. maybe it won't hurt from time to time.... If needed!
Hint: The highest octane the petrol is , more difficult to get mixed with other fluids it is!
P.S. I have been a bit of Yoda in this last sentence ... :lol: