Thing to remember here Kenny is the turbo sucks through the airbox not blows, so the more air it can get in the better.
SEAT wouldn't go to the trouble of designing a seperate intake for the PD160 if there wasn't a very good reason for it. It's not fitted to the Cupra 1.8t Ibiza so obviously the PD160 TDI needs a LOT of air. I guess that's because of the extra fuelling over the 130 to release the extra power needing extra intake air (bearing in mind a Diesel has no throttle butterfly) to keep the exhaust smoke free.
Tricks I learnt years ago when messing about with old VW GTIs (which had no turbos obviously) was that a long thin intake gave better low end torque and a short fat one gave better top end. VW were also aware of this and gave the later V6 engines a variable intake that switched between the two.
Lots of swirl is desirable in the intake but with modern engines the MAF and it's housing is the weak point. I would think a Cupra R/TT 225 MAF housing would be a good upgrade if you start making lots of holes in your airbox. Just my 2p worth and I'll bend over and get ready for the flaming.....
