Fifth gear tested it on a Golf GTI. Here's the difference it made:
Asda unleaded - 235.8bhp
BP unleaded - 236.1bhp
BP Ultimate - 236.7bhp
Esso Supreme - 240.5bhp
Shell V-Power - 240.9bhp
So yes, it makes a difference, however you really arent going to notice 5.1bhp on the road, and the differerence in something like a 172 is going to be even less due to N/A.
The main thing is that many cars are mapped specifically for super unleaded, especially modified cars. If I run my car on std fuel it runs like a bag of nails, however before I had it mapped std fuel was fine.