One comment that I'll make about using higher than needed octane rating petrol is, I have read, and I've also found it to be true, that when using higher RON fuel in an engine than it can advance the ignition to make the most/best of the extra RON fuel, will lead to reduced MPG in general.
I'm backing that up with the period in time that I used 97 petrol in my wife's previous 2002 1.4 16V Polo 9N, that car at that time needed to be run on 97 petrol as it would ping/pink on anything less - that was eventually found to be caused by it burning too much engine oil due to a gummed up/faulty oil cyclonic separator which was having the effect of lowering the RON of the incoming fuel mixture (port injected petrol and air borne oil mist). Yes the low end torque and general driveability improved quite a bit, but the MPG did slightly drop, it was Shell Super Duper stuff I was using.
In general I'm not knocking the use of 97 petrol where it is not needed, just pointing out what the outcome can be, ie reduced MPG and along with the higher price per litre, an overall increase in £/mile.
Nothing wrong with loading up with Shell Super Duper - ie a complete tank fill, to try to improve the cleaning of the engine internals, except the inlet tract area, I try to plan doing that every 6 months but only in the interests of trying to keep the engine slightly cleaner just in case my normal 95 petrol is not loaded up with enough cleaners.
I think that the rational for this drop in MPG on engines that are not able to make best use of that RON of fuel is, the final few RON points are achieved by dumping in a component that does have a lower "energy" capacity than the base petrol (wrong terms being used there, but I'm just trying to provide an idea as to why lower MPG can occur), in the distant past, UK based refineries, I think, used to distill the higher octane fuels "higher up the stack" and so they had a higher specific "energy" capacity than the lower octane ones, but globalisation has meant UK based refinery owners that still did that saying, "sod that, we will just do it the same way as our continental cousins do it and save loads on money" - and so we are were we are right now!
Actually probably only a couple or three non oil companies own and run all the worlds refineries right now, so they care only about money and not what would be best for our cars.
Edit:- sorry, I should have asked, what age and mileage is that car, you do know that DI petrol engines do coke up their inlet tract with hardened/burned on oil deposits, and eventually that will affect the power output and the MPG, the only answer to that is to get the inlet tract, ie the cylinder head inlet valve area shot blasted with walnut shells. Certain usage/running conditions seem to accelerate this build up more than others.
