I'd say custom... Quiet enough around town to be discrete and not getting pulled over by the police, but give it some right foot and mmmmm italian grunt!
my old car (slippy's car now) i paid 140 for a janspeed backbox, then about another 40-50 quid to have the centre pipe taken off and replaced with a solid bit of piping (so removing centre silencer). and it sounds something like this
(please ignore my gay little 'hello' half way through, i think some old woman was doing something silly on the side of the road. pps, no speed limits were broken)
go for a powerflow mate, i know there is lots of talk about their dodgy workmanship, but i got one done from cat back. since had it up on my ramp, and all the welds are spot on, sounds really throaty when you give it some right foot, but just nice when your driving normally.
if you dont want it to be too loud, just go for a back box. if you get a full system tehy will probably just weld a straight pipe from the cat, removing all the silencers - making it louder.