Don't know a lot about these little guys, but the big guys (Detroit, Cat, Cummins) sound pretty raspy with straight pipes. Any restriction you can remove downstream of the turbo will help performance somewhat at the cost of being louder. If it were me, I would do a turbo back system of a larger pipe diameter than stock. The larger pipe will smooth out the sound and if it's still too loud you can add a resonator to quiet it down a little without restriction and reduce droning. Many big rigs run 6 to 10 inch straight stacks and are not terribly loud. Next time you see a big ol' Pete coming down the road with big pipes, pay attention to how it sounds. In general, the bigger the pipe the deeper the tone of the exhaust.