Ditching an aftermarket muffler will not give you many gains, as most are straight thru flow mufflers that make good power. I would keep the nice stainless muffler.
If you have stock exhaust, you might pick up some power if you ditch the muffler. A nice cat-back system is the way to go for the best sound and power however. If you want louder, get an exhaust dump...may have overboost problems on some trucks with the dump installed tho.
I think you'll be very happy with the ATR exhaust system.
HTH!
Tyler