I should also add that during those 170 miles no TDI's have passed me. Come to think of it nothing has passed me in those 170 miles. Last night a new A8 4.2 quattro tried to shake me but he seemed to have a flat spot or a problem with his car as I easily passed him.
I guess that a 4.2 v8 would do something similar if it were to be driven similar. I used to has a 1.8T A4 2wd and it also got similar mpg when driven hard permanently. I had it remapped to around 200hp and the fuel economy stayed the same but performance increased. Personally I didn't but my A6 2.7 bi-turbo to save money on fuel, I'd have bought a TDI or something slow like that. I've also had an AmD remap. ( 302hp and 321lbs of torque and it's great ).
Back to original question though. I would look for something with less miles and make sure it has the full service history. My personal feelings would be to get something like mine. Xenons, leather, auto tiptronic, parking assist and a metalic paint. If you can get one then the Bose and Sat Nav option can't be bad. Run the car for a while then get the remap, I've had no problems with it and the power is amazing. Few blokes at work have bmw 540 sports and it's as quick as them no problem, except it doesn't have the v8 sound. The traction from the quattro is very good also, the only down side is if you drive fast on open roads then the fuel figures are low. I guess even a TDI driven hard all the time is going to drop to mid 20's, but then you'll smoke a TDI easily, especially after the remap.