I really don't think the cat back has much to do with torque. It's not the restriction in the system, and making it bigger would have about the same effect as putting dumps on a car.
The H pipe/X pipe and headers will affect torque more.
The X pipe, from my understanding offers a shallower, more European tone, and it can sound pretty funky sometimes. The H pipe is deeper, and it's probably more versitile.
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