Here's the standard downpipe - very restrictive with its 2" outside diameter!
And the replacement decat - mmm, shiny:
And the result? The sound is a little deeper on idle, not noticeable when cruising at normal speeds, and definitely deeper when accelerating. And it's definitely made a difference to the way the car picks up.
The resonator in the downpipe started to fail after a few weeks - though at first I didn't realise where the rattle was coming from. After about 8 months it got so bad that I had the resonator cut out and replaced with a straight pipe.