Thats some good stuff Olddog, now wheres my dime?
Some cars can take advantage of the higher octane, many can't, if it can't your wasting your money. Cost/benefit analysis thing. Computer controlled high performance cars, generally speaking, are more apt to take advantage of the higher octane due to the ignition timing being advancing more by the computer. More advance means better mpg, which (in my opinion) is the primary reason for 'runs better' and gets better mileage. I suspect the higher BTU rating potential advantage is not really a factor. A lower compression engine with static timing may well see no benefits at all.
I experimented with higher octane on my motorcycle, carefully checking performance and mpg gains, nada, I got no difference, but certainly spent more money.
The bike is computer controlled 'everything' (even the automatic transmission). Late model Suzuki with fuel injection, electronic timing, high compression, etc. I get 43 mpg, it's never changed from day one and it's got 30,000 miles on it now.
This advanced timing thing also makes a strong case for a vacuum advance distributor for increasing mpg on an 'old school' carb type motor. But be careful, there is a risk advancing it to far under cruise conditions and when your all ready pushing the envelope on the compression ratio. As many of us are...