Is your GTD Estate a 6 speed DSG or 7 speed DSG?
I have a 16 month old A4 190 TDI Quattro Avant with DSG which is 205kg heavier than a 6 speed DSG GTD Estate.
On paper it is 53.3mpg combined. Longest run done was 190 mile round trip to Leeds with a 1/2 hour break in the middle, and got 57mpg maintaining about 75mph on the A19 and A1. On my 31mile each way commute (I only do it once a week, work from home 4 days a week) its around 50mpg in the Summer and 47mpg in the Winter. Around the doors, (journeys under 5 miles), I'm lucky to crack 40mpg. If the DPF is filling up, I'm down about 10% on those figures before a regen).
The 7th gear on the A4 only really comes into play if you're on the flat above 70mph, downhill above 60mph or Uphill about 80mph. Due to those thresholds (determined by torque I'd imagine), my A4 is more economical at 80mph (when it is almost always in 7th) than 70mph (when it spends about a quarter of its time in 7th).
Do you do many short journeys? I sometimes think that having that 7th gear could be counterintuitive to keeping the DPF clear if the engine is running cooler sat in 7th doing 1600rpm at 80 rather than 2200 rpm in 6th at 80, the car does seem to actively refen far more often than my 2013 GTD ever did.