To expand on what snrub wrote, if an engine is experiencing detonation, higher octane fuels will likely result in better mileage/power. If detonation is not occurring, there will be no mileage/power increase due to switching fuel.
Alcohol is hygroscopic, and will "attract" water. This doesn't mean you can set an unopened gas can outdoors overnight and gain an extra gallon of liquid in the morning, but yes it will phase separate and can leave your fuel container with a layer of gasoline sitting on a layer of alcohol/water, if allowed to sit undisturbed for a long enough period of time. Two cycle engines often use a weighted fuel filter in their fuel tanks, to allow for all orientation usage. Think chainsaws, and how they are turned several ways during operation. If your saw has been sitting, and you grab it up and go, that weighted fuel line may be sitting in pure alcohol, with no lubrication whatsoever. IF you can get it to start that way, a scored cylinder/piston combination is likely the result. Usually they won't start, however.
When you pick up your gas cans or saws, give em' a shake before putting them to use.
