As Fox has said, I have a 530d, so not too shabby. I also aint the slowest driver (but neither am I the fastest). I do a lot of motorway driving though (usually over half of each tank), and I find that sitting at 75 on cruise all the time isn't actually the most economical (65 on cruise aint bad, but dull as dishwasher. Then again, closer to 80-85 not on cruise seems not too bad either).
In fact, I was on a run today, filled up just off the motorway, and was motorway pretty much for the next 160 miles (round trip to Ikea, so there was a little off motorway / retail park, then took a more scenic route on the way back, so another 50 miles of A road / passing through town), and averaged around 42 mpg on that route.
Now, considering my old mans 320d only averages around 47 (about the same as my Passat 1.9 TDI), so a drop of 7 mpg (roughly, over the last 4k, of 9k total on the car so far, my 530d has averaged over 40 for every tank) for the extra horses / smoothness of the 3.0 is a worthwhile trade off.