Yes, because ethanol brings additional oxygen into the combustion chamber it produces more power because it boosts the amount of available oxygen. OTH, ethanol contains 1/3 less energy so the mileage is less. Mileage is pretty much directly coupled to energy content so lowering the energy by a third should lower the mileage by that amount -- EXCEPT that ethanol has an octane equivalent over 120 which allows it to run without problems in higher compression engines.
If you were to run E85 (85% ethanol) the car would have noticably more power but would probably get about 29% less mileage -- running in a 12:1 engine like the Mk3 FF may offset this somewhat but I'd still expect the mileage to be at least 1/4 less than pure gas.
E10, OTH, is not going to see much of a power boost nor much of a mileage loss...
The good thing about ethanol, even for those that hate the idea of it, is that you can run E85 when you want more power but not have to do any modifications to the car that hurt resale value. Of course, your car needs to be certified E85 to do so safely.
Lastly, some complain about the problem of water contamination with ethanol, but remember, most of the fuel treatments designed to rid your tank of water are in fact ethanol or similar alcohol. Running E10 most of the time as most of us do should prevent water build up and that's a good thing...
2012 FF Ti HB Sonic Blue, Built 5/7/12, 401A + Parking Technology Package and MFT w/NAV 3.0.2 -> 3.2.2 -> 3.5.1 -> 3.6.2