A few random thoughts:
I think the new Ford Focus Ecotec 1.6 turbo and the Hyundai new turbo ranges are designed for US low quality fuel and they claim quite high outputs and are able to run on 91 Octane. Goodness knows how they do that.
Apparently all Australian official fuel consumption tests are required to be run on 95 Octane (not sure what is done with regards to those cars requiring 98 Octane?).
Finally, I have yet to see a properly run test that proves there is substantially better performance running an unnecessarily high octane.
I am excluding those dyno tests for tuned engines.
I remember the old Shell tv adverts of my Pommie youth where car "A" with Shell always ran 5+ mpg better than car "B" with the competitors product. The advertising focus now is more on the better detergents in the higher grades will keep your engine running cleaner/better/more economically. Might be something to do with more truth in advertising required these days (Fatblaster apart)?