In the real world (as opposed to the EPA testing facilities), the 1.4T engine benefits to the tune of 3-5 MPG by increasing octane from 87 to 91. The 1.8 engine also benefits, but to a lesser extent. Both engines provide more low end power with higher octane. The easiest way to determine if you need to run a higher octane is by paying attention to the throttle feedback. If it's pulsing, increase your octane. The pulse is the result of the ECU pulling timing to avoid knock.