Actually it's the other way around as far as octane ratings are concerned, higher octane is a slower burning fuel, low octane burns up fast, hence you get pinging or knocking using low octane fuel in an engine designed for higher octane since the fuel burns up too quickly and produces power when the piston is still finishing the compression stroke. Using high octane fuel in an engine designed for low octane shouldn't cause it to retard the timing (it should advance it, if anything) but the catalytic converter may be working harder to clean up the exhaust of unburned hydrocarbons.

 

Definitely not a scientific study, but on a recent road trip I was "forced" to use 89 octane in my HAH. (It was cheaper than the 87! ) I went from getting 37 mpg to 35 mpg. The roads seemed the same, as well as the ambient temperature, and elevation changes. But, I have no way of knowing for sure if the higher octane is the cause of the 2 mpg drop.

Also while on this trip, I used 86 octane with no noticeable change. One station even had 85! I stayed away from that stuff. The HAH is rated at 86+.