Custom.
Mine's very quiet for 2.5" setup, but you could do the same with something smaller even.
Its a Dynomax super turbo muffler (I think I payed $50 cdn.), a 2.5" cat, and a nice long resonator. All bent to look competley stock.. It looks like a stock peashooter, but big piping.
Its a very quiet setup, and it was cheap. I love it.