Your 2001 car will already have a K03S turbo, a bolt-on K04-001 turbo wont see any gain over your stock item, both produce similar figures when remapped/chipped. It's more an upgrade for the earlier (pre 2000) cars with the K03 turbos.
The bigger K04 from audi TT/S3 and Seat Leon Cupra would give a big power increase, but fitting it is anything but simple. For a start it points in the wrong direction ! If you want to spend the kind of money for that, i'd just get an IHI roller bearing turbo or garret GT28r instead.
