Desiel or petrol?
Some people have done it with the desiel, no doubt they will be posting a reply - so I won't spoil their thunder and they will be able to go in depth about whats involved.
Petrol version, I have no idea, looking into it at the monent, what BHP figure do you have in mind? APR in America have realease a Stage III + turbo kit for a 1.8T Transverse pumping out 340 horsepower and 320 foot-pounds of torque at the flywheel on 93 octane gas.
At Awesome GTI (APR distrbutors) they doing K04 upgrade thats 245hp/275lb-ft on 93 octane gas, but think that can be remaped.