80 mph+? Well there's your answer. I did exactly the same journey before Xmas in my 140 DSG Kombi and got 725 miles before I needed to fill up, and then couldn't get more than 60 litres in. I drove at bang on 70 in the UK and 75 in France.
On the way back, I set the cruise at 85 an got nowhere near those figures, but I had to be in London for a job. Anything over 75mph in these things is just a killer, and I'd like to bet in a twin turbo, the second turbo will be spinning and sucking up the juice.