Well, here is my latest fuel economy. I've been doing quite a bit of driving and have noted VERY consistent fuel economy numbers.
I have a flat range of road ideal for checking stable fuel economy.
Thus far Here are my consistent averages:
65 mph with AC on, 57.4 mpg
70 mph with AC on, 50.1 mpg
Keep in mind, this is the 4Matic E250 (All wheel drive).
I did a road trip in my 2013 Passat, and the Passat despite 2 wheel drive and a lighter car is very close to what the Mercedes accomplishes in terms of fuel consumption.
Regarding GDI, the term for German cars is FSI (Fuel Stratified Injection), and it's been in use for well over 20+ years. Its extremely reliable, but not tolerant of poor quality gasolines. Use low grade gasoline without high quality detergents and your will cause SEVERE damage to the systems.
When using high quality premium gasolines, they are very reliable and efficient.
The Japanese have struggled with durability because of owners in this price point disregarding the requirements for premium gasoline, and these are the same owners that use conventional oils instead of synthetics due to perceived savings.