FYI, I had to put a baffled flange in before AND after my cat as well as a silencer in my cannon muffler to get over the pits.
That was on my Legnum with 2.5"/2.75" front pipes into a 3" catback, no resonators etc. Granted the 6A13TT was louder then my old RB25DET NEO but it still took quite a bitb to shut it up.