Higher octane fuel contains more additives, which, in addition to raising the octane level, helps keep things cleaner by their detergent actions. But, in general, using a higher octane fuel than the engine was designed for doesn't really pay you much as opposed to too low of an octane rating which can damage an engine. If you're lucky, you can buy gasoline without added ethanol, but there are a number of states where that is impossible. Ethanol lowers the energy content per volume, meaning that you'll take a hit on fuel economy. And, because ethanol has an affinity for moisture, an open system (the REx's is closed), can be a problem.
2011 535i x-drive GT, 2014 i3 BEV