It's a common misconception that higher RON fuels are significantly more calorific and therefore give you significantly higher mpg. If your car is set up for RON 95, (which pretty much any current petrol VAG engine putting out less than 270ps is), you're just chucking money away.
The higher RON is required on the likes of the S3 because the fuel quantities required to generate 310ps could not be delivered using RON95 without pinking.
Higher RON fuels are less volatile, so you can add more in per ignition cycle without pinking, it's nothing more than that. A higher RON rating fuel is achieved with adding Benzene, Toluene and naphalene related compounds in small quantities (around 0.1%), and these additions add almost no extra calorific value to the fuels.
Most RON 97-100 fuels only add 5% ethanol, rather than 10% and as Ethanol only has about 70% of the energy density of petrol, the RON95 E10 is about 1.5% less energy dense than RON98 E5, hardly noticeable.
To claim an increase in efficiency in the region of 10% through use of higher RON petrol vs RON95 is pie in the sky, unless you are driving with more care because it has cost you more. You can buy RON enhancing fuel additives that end up costing the equivalent of 3-5p per litre, that you add to a full tank of RON95. I don't bother because the price differential between RON95 and RON97 at my local Costco is quite small. If I were paying 15% more for premium fuel, I'd go down the RON95 +additive route for my S3.
