If you have a scout through the GTi bit of the forum there are a few threads on induction kits - I think I asked about it not long after signing up! General opinion at the moment is that a regular induction kit soaks up too much heat from the engine bay, therefore
reducing power
What you could do with is a shielded induction kit, which is why us GTi owners are all buying BMC induction kits

however there are alternatives, both Pipercross and K&N are developing kits which should work much better on the Mk5 - for a cost!
I dunno about the lower-powered models but certainly on the GTi you'd not gain anything in performance, but you would on the other hand get that nice induction "roar"
HTH?