I am running nothing after the s-shaped turndown after the turbo. If somebody says it's not "much" louder than a stock muffler, they are BSing you. It IS alot louder. It is within tolerance of most people, being diesel, it's quiet when you are not on the pedal. When you get full boost PSI there will be a fairly loud poping sound as some exhaust gets past the wastegate unrestricted.

As far as the legal issue, I live in a outer city/suburb area and have driven past many speed traps, cops on the highway, and cops at stoplights without no more than a glance at me. Since I am living here as a "student" I have not gone in for a Colorado inspection, so I don't know if they would fail me for it.
Oklahoma was no problem since they don't give a rats behind whats on the road, or what condition it's in.