True exhaust tuning the old skool way. Get a O2 sensor plug that fits the Sprinter threads. Drill a tight hole in it to support a press-in steel or aluminum tube, about a foot long. Connect rubber hose to tube. You will need a low pressure gauge to attach to the hose. You need to be able to observe about 0-15 PSI range if possible. Now with the 02 sensor removed , you can test drive while watching the amount of exhaust back pressure produced in the system. Unplugged 02 sensor will set a code.. but this is a temporary test. This testing is done at WOT wide open throttle, of course. Make sure your pressure testing rig has no leaks in it, duh... we will eventually be working with low pressures, hopefully. So say you have old cats and see are getting in the 6-8 psi range of WOT backpressure, your performance and mileage and everything else is suffering. You can then find the source of the excess exhaust restriction and fix it. A low restriction exhaust will cause faster turbo spooling as some have mentioned, less gear downshifts on hills, and better fuel mileage, if you had an accurate way to measure it (most dont). A straight exhaust might run, say 3.3-3.6 PSI just to show what a true low restriction exhaust looks like. Also, on a long Sprinter, the big muffler doesnt seem to reduce noise much. The turbo is what reduces most of the exhaust noise. A long tailpipe reduces the noise a bit. The cats can turn out to be the biggest flow restrictor. A plugged cat will mess up your backpressure, performance, self-esteem, etc ... no good.