Exhaust Theory?

[Poodle]

The bloke you talked to is dead right. Anything HDi this side of 180bhp will be fine with 2.5", get a 2.75" downpipe and cat if you're concerned. Going too big can supposedly allow for some strange resonant frequencies, plus it'll be heavier and sound like a wet fart due to the total lack of gas velocity. Look at some of the petrol turbo exhausts, they've got be heading for 300bhp before they go above 2.5".