Whilst I 100% agree with you MH, that VW's advertising was at best "creative" and at worst a damn right lie, TBH, I came into the GTD with my eyes wide open. Despite the lofty claims made for over 60mpg combined and nearly 70mpg extra urban (DSG Model), I never for one second ever thought I would get anywhere near that.
My yardstick so to speak was my previous 140 CR Scirocco (like you using your 170 CR Scirocco). Whilst it seems it wasn't as frugal as yours (but that could just be down to my journeys as my trip to work is only 10 miles ish), it averaged about 46mpg over it's 20k lifetime. I always said if the GTD could get that (or very near it), I would be pleased, especially as it's much more powerful than the older car. Well, over the first 1300 miles, the MFD is showing 46.2 for the lifetime (which equates to a real average of 45.3 working it out brim to brim), which is pretty close. Bearing in mind the car is still running in and feels tight, and my Scirocco did improve by 3-4 mpg once it went over about 5-6k miles.
The way I look at is that for a car with 184ps, capable of over 140mph and 0-60 in around 7 seconds, to get that and get mid forties MPG (though obviously not at the same time) is still doing pretty well. The GTI boys were promised mid forties and most of them are struggling to crack 30-35mpg, and in the real world (and I'm going to be shot for saying this), if you are not doing traffic light standing 0-60 drags, the performance difference between a GTI and GTD isn't that huge.
We could analyse this all day - is it tyres, drag coefficient, engine map, the fuel type etc, but my guess is that the 170 CR in the Rocco and the Passat probably over specified in HP, whereas the 184 CR in the GTD, probably only just meets those figures (of course I could be totally wrong

), and is probably more to do with new Euro Emission Laws and the newer cars needing to be greener and emit less CO2 etc. I'd probably bet that if you had a stock 170CR Scirocco and a Stock 184ps GTD on an identical rolling road, they would produce very similar power figures ?