The first few hundred miles will always give you abnormally high fuel consumption. After 3000km, I am averaging 35mpg which, TBH, I am slightly disappointed with, considering that I drive quite carefully. The thing I have noticed about the GTI compared to other cars how consuption increases wildly when I have a little bit of fun 
But you have yours re-mapped!
You can't expect to make an additional 40 BHP and not burn fuel doing it.
GTI is like every other car...drive it like you are taking your Gran to church and you'll see late 30s MPG...but what's the point? Buy a diesel.
Drive it properly...and you'll see 20 something.
Them's the breaks...I think my Test Drive average was 20ish MPG - a figure I wouldn't expect to do much better than in normal usage, if I'm being honest.
I only get about 40 MPG out of my smelly-fuelled Fabia vRS despite everyone telling me it would do 55-60 MPG.
I would expect a GTI to deliver 20 something MPG is you use it 'properly'.