I had a s/s Piper cat back (3.5" outward roll tip) on my Mk1 1.6 Focus (zetec-s engine). Rolling roaded @ 106bhp with it, but obviously that depends on the car. Cost about £290 fitted back then (2004 I think it was). It was a subtle tone when driving normal then louder at higher revs. It wasnt deafening either.

Andy