On my VTS, the 3" tailpipe Piper with decat and middle box sounded superb, but was too loud for me. Swapped it for a 2" tailpipe backbox and it's much less intrusive around town - perfect imo.
3" tailpipe Piper with the CAT left in isn't too bad.
www.manicmotors.co.uk are suppliers of Piper exhausts - give them a call.