With the 1.8t i could see how you would feel the difference for sure. But with some other motors and that fact the ECU can change to meet the demands of lower or higher octane, i think its a fable.
The computer will absolutely retard the spark and reduce boost when running low octane fuel. This will yield less power, and - in my experience - poorer mileage, though some people dispute the latter.Am I imagining this or is there a real world difference to the average driver ? Does the computer retard the timing when using lower octane fuel or ?