It's a lot easier and cheaper in the long run to buy the 1.8T.
The best mods for NASP car engine are as follows.
Internal work yields the biggest gains, head work like porting polishing and 3 angle valve jobs will maximise the airflow and raise the power.
Bigger valves are an obvious addon but not all engines are suitable as it depends how close together the OEM valves are.
A fast road cam will yield good power gains, especially in the top end of the rev range. Don't go mad though a profile of up to 270 degrees will give the best gains without the engine becoming cammy.
The most popular power mods are induction kits (which are questionable as power mods but they can sound nice if that is your sort of thing) and sports exhausts with sports catalysts or decats.
To keep the car legal I'd recommend a sports catalyst rather than a decat.
Total cost of all these mods will run into the 1000's for a power gain of around 40-75bhp! Get a 1.8T instead, remap it for £300 and you have an extra 50bhp easy!
It's worth having a read of the articles on
www.torquecars.com/tuning/ for more information on these mods and what the terms mean.