The theory is a smoothed airbox will flow air better, theoretically it sounds good but on a forced induction (turbocharged) engine the air is being sucked in so hard by the turbo it going to get the air it needs anyway as long as it isnt too restrictive.
On an NA engine it would probably yield better results as the air needs to be sucked into the engine on each piston downward pre-firing stroke, this is why they use cams on an NA more than on turbocharged cars as the air is forced in by the turbo anyway so you dont need to work as hard to get it in, where cams keep the valves open longer or with more lift for more open area to allow the amount of air needed to be sucked in.
I am going to do mine though every little helps as they say and I bet that plastic weighs a few grams !
As it happens TDi 110bhp remapped, but i smoothes mine the other day going on what i heard i had spare time so spent a few hours doing it all... Did take a few hours as said to do it proper and very fiddly but seems to have make a difference!!! After removing all the plastic in the way i ran sandpaper over it didnt go to town just got it quite smooth didnt 1500 it just got fine sandpaper... But yer as said seems to have worked a bit for me...
BUT seriously its painstaking wouldn't wanna do it again lol !!!