Nearly everyone that drives a deisel these days that is interested in better fuel mileage and more power are doing two things first...replacing the stock intake with a cold air intake and removing the stock exhaust and replacing it with larger pipes. In states where it is legal they are usually running straight pipes from the turbo back, eliminating the catalytic converter and muffler. Once they have adequate "breath" for the engine the modifications available are nearly unlimited...larger injectors, intercoolers, electronic tunig package, propane injection, etc.
The next time you are beside a deisel truck and can distinctly hear that whistling whine of the turbo you can bet he's not running a stock exhaust system, and if black smoke appears from the exhaust when he accelerates hard he's also running aftermarket injectors.
I don't like the vertical stacks, but I am working on some of the other stuff as we speak for my '96 F-350.
