There are many advantages to using super unleaded, and the mpg improvement is only a minor one.
It is a much cleaner fuel, which causes less wear to the engine due to it lubricating the block better.
There is a noticable performance gain, best being shell vpower proven on numerous occasions.
Also during the tests done, no-one knows how long they ran the cars on each type of petrol, so the ecu might not have been able to adjust properly.
I have used sheel optimax and then vpower for many years now and can tell you that its well worth the money.