As someone who recently swapped from a VAG TDI to a petrol A3 TFSI I had the same question. The DADA type engine has Cylinder on Demand which effectively turns it into a 750cc twin whilst cruising on part throttle. You can't feel this happening as the transition from 4 to 2 cylinders and back is imperceptible but it aids fuel consumption.
I am getting 50mpg plus out of town and low 40's around town. These numbers are 2 or 3 less mpg than I was getting from the (now scrapped thanks to the Mayor) 2003 Bora TDI 150.
There is a slight hesitation at around 1200 rpm when pulling away so it needs slightly higher revs when moving off. I believe that this is well known and maybe less of an issue if you don't have a manual gearbox.
