That I can vouch for, cousin has a Cruze CDX and it states to use only E10, he thought he would put 98 in it one week to see if it does any better in the fuel economy, he ended up getting almost 200k's less to the tank, if you were to retune the ecu's timing then it would run on 98 but at what advantage.
I have looked into running premium ethanol again but (old seca used to love v-power racing) but they state not to run ethanol contiuously in a car that was not designed for it as it will perish rubber, this may not be true but I would rather not find out the hard way.
The apparent reason that ethanol was ruled out over here a while back was because they were introducing too much into the fuel, they have now found a safe level.