Hi Gary, i had a k&n 57i induction kit on my tdi 150 and i never felt any power difference at all the only thing that i could say is that it made the engine alot louder, it had a bit of a rumble on tickover which wasnt a nice noise it just amplified the diesel sound but when you floored it the rumble went and it just whistled and wooshed alot as the turbo comes in which i thought sounded pretty good.
Is it worth it? well i got mine second hand off ebay for £40 which was good but i'm glad i didnt pay £95 or whatever they are now cos i think i would have been dissapointed. i woundnt buy another for a tdi,they just dont sound the same as they do on the petrol cars.
Hope that sheds some light!
Nick.