If its setup on unleaded, you won't see the most of the gains available by using super unleaded. However, if it's mapped hard on super unleaded, and you use just unleaded, detonation will occur much earlier. I had an engine at work that was mapped hard on 98 RON, we put it on 95RON and had to completely remap it, as it was detting everywhere above 40/50% load. Which could easily represent you driving up a hill. And if you come on boost, chances are even higher!

As said though, depends what fuel it was mapped on, and how hard it's mapped. I know for 10p/litre I'd not be risking it!

The good news however, is detonation is the sound of fuel economy