Hi can I add my bit to this discussion. I have 2010 IS 250 my local garage only has E10 so I have been using this for some while with no obvious effects. A 200mile trip down the M1 averages about 40 mph around town about 30mph.
The paragraph below was taken from Wikipedia 'Ethanol Fuel'
Ethanol contains approximately 34% less energy per unit volume than gasoline, and therefore in theory, burning pure ethanol in a vehicle reduces range per unit measure by 34%, given the same fuel economy, compared to burning pure gasoline. However, since ethanol has a higher octane rating, the engine can be made more efficient by raising its compression ratio.
My guess on why pre 2010 cars are not suitable for E10, is that on 2010 cars the compression ratio has been raised to compensate. However on pre 2010 cars with a lower compression ratio the published fuel consumption figures wouldn't be correct when using E10, therefore opening up a can of worms for the manufacturers.