However, I've also seen that if you put in a new air filters, induction kits and what not you can boost the performance for a lot less money. Are these claims realistic? What kind of cost is involved? And what kind of bhp gains can be had?
Dont bother.
Only the downpipe is restrictive on the 1.8T ,but not until you get to ~240bhp. (You'll have replaced the turbo at this point anyhow)
Get the car on the rollers first and have it properly set up on standard settings.
I can guarrentee that it will be down on factory output due to a soft wastegate spring (heat cylcing weakens it) and that the unbelievebably poor plastic recirc valve will be leaking what little precious boost you already have away.
An easy 150bhp is possible on stock boost and 190-200 piss easy on a properly set up base car + stage 1 chip.
Obviously the engine needs to be properly serviced with clean filters, in good health, and with fresh plugs and leads first.
Tim..