1. How well do the stock cats flow as compared to a high-flow cat?
2. Do i need a centered muffler or offset if I am replacing the stock one?

 

I am not for sure on the cat question, i have a 6.0L and the cats are different than a 5.3L. The mufflers I have are center in and center out, both are vertical mounted.

 

the stock cats are probably pretty restrictive, they certainly arn't performance cats, if there even is such a thing.

The kind of muffler you can go with depends on what you want to do with it. There's a lot of room under a truck, so an offset one can be made to work pretty easily. Not to mention the stock muffler is HUGE.