I went and had a chat with the local exhaust shop as i needed to get a euro 3 cat for the emissions test, using a rough guestimate on required materials i can make my own custom 3.5" catback with two large mufflers for about $1000. Thats assuming i can fit decent sized off the shelf mufflers under there.
So for about the same money as a jap catback i should be able to get pretty close to legal DB.