Further to what I posted up before I have just come back from holiday in France, and have with a full car managed to get over 400 miles out of a tank of 98 RON on two occasions this was supermarket fuel as well so nothing special. With about 100 miles of N roads at 90kph followed by auto-route up over the massif central and down across the Millau viaduct, which takes you up to over 1,000 metres altitude on 3 occasions, so I'm fairly chuffed with that.
The stability of the car at speed is also better so even if you dont feel comfortable with the larger engine tray Robin the rear control arm cover off of the TT and the rear axle cover would be a good investment ~£40 of parts which will reduce the air pressure under the rear of the car and wont affect ground clearance at all.