The numbers say that ethanol free regular (87 octane) gasoline has about 2% more energy than an E10 blend. So you should see roughly a 2% improvement in mileage by switching from E10 to pure gasoline. That's less than 1 MPG in my case, and given the normal variations in my mileage I haven't really noticed a difference.
BTW, 91 octane premium actually has slightly less energy content than 87.