I really can't believe those Autocar results. Unless you do consumption tests in a controlled manner you won't get reliable results, I come from 30yrs in engine development and you really do need to control conditions very accurately. The difference in calorific value, stoichiometric A/F ratio and density is of the order of one or two percent, and you need very carefully repeatable conditions to measure that sort of change. I don't know how they did it, but coming up with numbers like 10% makes me think it was some sort of back to back road drive.
In a modern car going from E5 to E10 really won't affect either the perceived performance nor the general fuel economy, unless you can spot a 1 or 2% change (most people only notice something like a 10% change in power, anything less and it really isn't very obvious in everyday driving). I've used E10 on the continent quite regularly, it's fine.