a part does not add a certain amount of hp.
if it were that easy, you and I could buy our way up to 200 whp like Leggo blocks. Reality does not work that way. Please lose the newbie way of thinking: i.e. what's the next part to get and how much will it add? that's pretty creative hp accounting. well, to answer your question, if you get the wrong design for that, it can lose power...that's the shocker isn't it?
The exhaust can assist header and cat generate higher flow...that's what makes more power. You size the exhaust SYSTEM to the hp goal. guess what? the exhaust is only 1 part of the exhaust system. if you buy one without considering the size you need and how it worls with the header and the cat, you can undo all the gains made by a header and aftermaket cat.
systems ...think about it....