Back in the '50s major oil companies ran ad campaigns promoting the amazing benefits of high octane gasoline. I doubt they ever dreamed those ads would create myths that would last for generations. Even the story about the 100 MPG carburetor oil companies paid to keep a secret eventually faded away.

Pure gasoline has roughly 114,000 BTUs per gallon. E10 has a few thousand less. If an engine is running properly on fuel with enough octane to prevent detonation, it is physically impossible for a higher octane fuel with the same energy content to somehow 'find' additional horsepower.

Maybe we need to think about it this way. A modern engine is designed to generate a certain amount of horsepower. If you feed it crappy gas, the engine management computer will detune the engine to prevent damage, resulting in less horsepower. When you switch back to fuel the engine was designed for, the engine will run optimally, and you'll 'get back' the power you lost with the crappy gas.