Duroo - what was your fill volume following those 412 miles? If it's something in the region of 45.8L/10 gallons, then that's 41mpg, I've never had to put more than 47L into mine, it seems to have about a gallon spare when the range in the MFD says "0 miles". It would seem as if your GTD is a little worse than your MK5: 88 more miles on an extra 5L in the tank. Assuming 45.8L gave you 412 miles and 50.8L gave you 500 miles, we're talking 45.1mpg, 10% better.
I generally get around 430 miles from a 10 gallon fill on average (43mpg, MFD says 44/45mpg for the tank).