Ideally, you'd want the diameter of the exhaust system to be the same as the diameter of the outlet on the exhaust side of the turbocharger (or larger). However, I don't think a stock MB diesel is hurting for a larger exhaust. The OEM diameter system is probably adequate.

Also, playing around with different pipe diameters/mufflers could cause unpleasant resonation inside the vehicle or simply make it too loud.

But, yeah, the sewer pipes they're putting on turbodiesel pickups are really cool.

Aaron