I got some tomei uel headers and i've been running e90-80 from a local gas station for the last 5k miles. Never had a problem and love e85
I decided I should put some 91 with some detergents in the tank for the hope of cleaning a little, so I ran the tank down to e45 with 91 from shell and was just driving around a little and the car seriously sounds different!
I normally pull from any gear around 3k rpm to get around cars but at e45 I need to pull from 3.8k-ish to hit the powerband. My guess is since the headers are uel and the powerband is different, the air is colliding at different intervals in the headers making a really different exhaust note.
Just a theory but I'm gonna get a good mic and test it to find out! :p