The stock exhaust after the y pipe is one of two configurations:
3" single stainless steel pipe into a single in/single out muffler with 3" single stainless, mandrel bent tail pipe.
3" single stainless steel pipe into a single in/dual out muffler with 2.25" dual stainless, mandrel bent tail pipes.
The maganflow cat back system is 3" single stainless steel pipe into a single in/dual out muffler with 2.5" dual stainless, mandrel bent tail pipes.
So the only real difference is you go from 2.25" dual tail pipes to 2.5" dual tail pipes, which is not at all necessary. The dual 2.25" factory tail pipes flow more than the single 3" pipe before the muffler, so there is no benefit to the larger tail pipes.
I don't see where the cost of the cat back is justified vs just replacing the muffler. It's fine if you want to spend more, but I'm just giving the original poster real information to save him money if he doesn't want to spend it where it isn't beneficial.
Breathing in and out is good enough for me.