Yea... 27.5 MPG US is certainly low. My commute is 1.8 miles. I've gone full tanks only doing that drive... to work, home for lunch, back to work, home after work. Not good for any car but especially the TDI. Still got over 400 miles on the tank, somewhere between 11.5 and 12 gallons when the light comes on. That's right around 33 MPG and I wouldn't expect to see much worse. Maybe could get it just under 30 in a colder state on winter-blend with a similar commute, but if I was getting 27.5 I would be concerned.
And yes, hand calculate. Once you have a good hand calculated baseline (several tanks) you can adjust the car readout up/down to closer match your real values.
The only thing Eco does in my car (manual) is mess with steering/throttle response. In your DSG is it possible that it's keeping RPM's so low that you're spending a lot of time lugging the engine (or giving it more throttle to force a down shift)? That might be why you're seeing worse mileage in Eco. Assuming hand calculations all around.
