Many places, premium fuel does have ethanol. Here, you need "recreational fuel" to have zero-ethanol premium. This gasoline is intended for boats, but also commonly used in lawnmowers and snowthrowers because its usually kept longer, and doesn't have the risk of varnish (which can clog up a carbureter) that ethanol fuel does. Our highest octane gasoline here is 93 in the Upper Midwest; it's not necessarily ethanol-free.A higher octane does not mean the fuel is more powerful. Octane rating in gasoline is fuel's ability to resist engine knock (pre-ignition). Premium fuel however does contain more energy per unit in general because it usually has zero ethanol.
I also use Costco 87. If I'm traveling I get BP or anything Top Tier. I did accidentally get Premium 91 once and didn't notice any difference.Mid-grade to most of the country is 89 octane. I know 85 octane is typical at elevation in Colorado but I would not run it in a car equipped with a turbo. The manual specifies 87 or greater. No allowances are made for elevation. I typically run 87 from Costco. I think I ran one tank of 91 and detected no improvement.