A re-mapping is what the GTI is crying out for. If you have it done it will take your breath away. There are a lot of different company's offering re-maps at present so do some resarch, ring a few of them up and see which you like the sound of.
I can't talk highly enough of Morego ( the 250 also includes a Miltek back box which is spot on

) though so I'd recommend talking to them. You should also look at break and suspension mods first. I had mine re-mapped first with the power grip and s''t my pants with the standard breaks. The AP's which I have fitted are also as good a mod as the re-map and well worth the money.
As far as the fuel economy goes when i take it slow it's around 30-32 MPG. Not hanging around in mid 20s, driving like a t"@t had it in low 14s. 99 octane is a must when re-mapped as it feels like the hand break has been left on with regular.
Is it a good idea to re-map with DSG? HELL YES! The increase in mid range torque flatters the DSG and theres no waiting around to change gear (in D)before your off on part throttle. In town on motorways and A roads I like just putting it in drive and riding the wave of torque, power, torque, power, through the gears. Theres not a lot of cars that keep up with you with the re-map/DSG combo. On B roads with the paddles the car feels so fluid and well balanced you get carried away. You find your self getting sucked in. Driven faster and faster which is why you need the big breaks IMO. It makes the standard car feel breathless.
Best place to do some resarch is tyresmoke. There is a wider range of options being used there by more people there.
Is it worth re-mapping? Yes Yes and further more yes.
Hope thats of help
