With stock injectors you will not likely see any gains from stock straight-piped to a 4" turbo-back system. The 4" is LOUDER, but I prefer the sound of straight-piped stock in my F350 (AC codes and 38R) over the MBRP 4" on X.
I would go with an mbrp kit I like the sound and there very easy to install I've put one on one of my less modified 01 and another truck sound great plus I think the exit angle looks mean