HI,
Just sold my MK5 GTTDi today (PD140) and bought the GTI TFSI as a replacement. As a guide, aim for 39-43mpg at most, I work in a service centre and regularly check cars trip computers, most see these figures, obviously depending on driving type.
Great cars in general, suffer from turbo faults a LOT (listen for a whistle that sounds like a faint police siren) and is a £500+ fix. I have done 5 in the last 2 months. Other than this they seem solid and reliable. Low power at low RPM is usually caused by faulty MAF sensor which causes a sudden surge in power when under load at 3000 rpm.
A remap is well worth it but avoid the cheap mobile guys, lots of rubbish maps floating about for these.
Hope this helps.