There is a very specific reason for this.
Audi lowered the 1st, 2nd, 3rd gear ratios for the DSG.
This has been done specifically to give an impressive 0-60 time.
The trouble comes when you hit 60mph, when 3rd changes to 4th.
There is a big gap, so it really struggles to pick up in 4th.
For proof, look at the 0-100 times.
The 140 may only take 8 or so seconds to 60, but it takes a whopping 26 seconds to reach 100.
That's 5-6 seconds slower than a standard 320D.
According to the DIS I've averaged 50.5MPG since I last reset it if that's to be believed.
Not 100% sure how much I get out of a tank I but it's in the order of about 500 miles before the light comes on.
The highest I've seen was 57 on a run from Plymouth to Brum.
On the other topic of the the TDi not liking it under 1500rpm, my 140 will happy sit below 1500 without complaining and is perfectly drivable from just under the 1000 mark. OK it doesn't pull like it can but for around town it's fine.