i say get a 3" dowpipe and leave the cat back as is.
wish i hadnt have added the cat ack to be honest. yes it sounds mint, and its not to bad on a long run, but when you accelrate o gid forbid are speeding, it is so loud, it is blatently obvious you have your foot down.
im going back to 3" pipewerx Dp mated to the standard back box as that gave it a nice deep burble that wasnt overpowering and souned way better than std.