That might sound good, but it makes no sense. Once the spark fires, it all burns. Octane rating only means it takes a higher pressure to ignite without a spark. It doesn't mean they added dirt and water to make it burn poorly :/ Higher octane gas might indeed have less energy per liter than say, 87, but the difference is so minimal you could make it up by cleaning out your glove compartment. A bigger issue concerning energy density would be oxygenated fuels used in the winter in most cities. Oxygenated fuel has less energy than non-oxygenated fuels, per liter, and you will get noticeably worse mileage.
AFAIK, the flash point of a fuel is unrelated to the amount of energy it stores. And octane has nothing to do with the speed at which the flame front propogates.
All gasonline isn't the same once you include additives, so one brand's 87 might knock, while another brand's doesn't. Every station in an area might get their gas from the same refinery, but use different additives to get the octane numbers they sell.