From what I remember (starting to become a dim and distant memory now) I don't think this stuff actually increases the octane rating, it works as a flame retardant. Not as daft as it sounds. As you increase the compression ratio in an engine it becomes more and more liable to suffer from detonation (or pinking), so adding some of this to your fuel allows you to either run a higher compression ratio or run it with more advance with the ignition timing. And up to a point more power.
I think it was this stuff we used this years ago to prevent detonation in a high compression race engine got it from a motorbike shop where they use it for two stroke trial bikes. I seam to remember lots of guys shaking fuel cans before they fuelled there cars....
So yes it is worth while if you have made changes to an engine, I seam to remember that when BP lunched its 102 octane fuel all the cars they tested showed an increase in BHP but only after they had had there ignition re-mapped. Seeing as how a manufacturer sells its cars knowing that it will have to run faultlessly on many different octane fuels so compromises it's set up is hardly surprising?