I know this has been done plenty of times on here, but thought I would post my experiences.
Since I got my 156 2.0 TS 2.5 years ago, I've kept a close eye on the consumption of fuel. I always reset the trip when I brim the tank to see what I'm getting. (I try to drive economically unless I need the power as I have a Westfield for guzzling juice in). In the early days I'd struggle to get 320 miles from around 55 litres mixed driving. After renewing the thermostat, and becoming more familiar with the way it drives, I found I was getting around 350 - 370 between refills. That's around 29-30mpg which seems to be the norm for a Twinspark.
However I tried a tank of super unleaded 97RON from sainsburys, where I always refuel (I know
) and found it immediately smoother accelerating. This morning I've just re-brimmed it after getting to 400 miles from my usual mix of driving. This in itself is a first, but better still, I only squeezed 46.3 litres in. That's around 39mpg.
Nothing else has been changed.
I'll continue to monitor and may try going back to 95RON for the next tank, but found it quite a remarkable improvement. As an aside, my girlfriend's TT 1.8T has also just swapped to a 97RON diet and feels smoother too, although she's yet to do MPG calcs.