I've got a BMC on mine, I would say it makes a huge difference but you certainly notice if you don't clean the element every 5k miles - the TDI's suck quite alot of air up so the filters get clogged up - I think air flow is more important than people realise with TD's
all diesels suck stupily hard!! On a mondeo st diesel a proper induction kit will get you 15bhp easily!! i run a KN pannel on my 115 and it does make it sweeter, i may swap to a full induction at some point, if i find one for sensilable money.