I have tried 97/99 octane petrol in many of the cars I have owned and in my experience it can make an improvement but only in cars with a turbo. I went through a couple of spells of trying it in my Jazz and each time I found the MPG to be no better and sometimes worse than normal, so for the jazz it was 95 octane only.
As for the supposedly extra cleaning additives/properties of super unleaded, I can remember when an engine would need a de-coke after 40,000 miles. With the fuels of today the word "de-coke" isn't even in the modern mechanics vocabulary.