i wouldnt think so because you still will have a reduced back pressure than the stock cat. the stock cat is a bit restrictive. A stock cat is also bigger than the piping. which in the stock exhaust cause would slow the gases down a bit. But since your gonna replace the cat i would imagine that you still would have free'd up a bit more hp since the gases will be able to exit much faster, than a stock cat. And because the piping is smaller you will probarly create the same amount of backpressure.

someone correct me if im wrong

the only way to be sure is to but the damn thing on already.