This may be unrelated, or may be helpful... either way, here goes!
There was a thread that discussed fuel economy investigations:
My Fuel Economy Investigation (Petrol) - burns863
And there were a few posts in there that did refer to differences between high and low octane fuels:
Wikipedia Quotes
So a higher octane rating means that the fuel is more controllable - i.e. less prone to early detonation due to high compression.
My understanding is that when using a higher octane fuels in a high performance car that adapts its compression ratio to the fuel in the tank, you'll get gains in economy and performance. However, if you put higher octane fuel into a car like a Robin Reliant, it will actually slow it down.
As for long-term health of the engine, I gather that a lot of companies put cleaning products in their higher-octane or 'premium' fuels that you don't get in the lower octane fuels (which are kept as affordable as possible). So it's really what else is in there apart from just what the octane rating might be.
Hope there was something helpful in there...
All the best,
Andrew.
High-Octane vs. Low-Octane Fuels 
