DCook,
The cat converter is way more restrictive than the muffler,but if you want to keep a muffler on the truck then ditch the cat and muffler and get a free-flowing aftermarket diesel muffler.
You heard it right... you will get better throttle responce by the turbo spooling up faster which in turn will give better performance,but the engine will not make more horsepower. You need to add more fuel to make more horsepower.
-Mike