Whats the real deal on catbacks?

[Dexion]

First of all why aren't there more catbacks available for the Sedan? As far as I know its just Stillen and Nismo? Did they forget us?

Secondly I'm still not even certain that an exhaust system adds any gains on the 3.5's I'm still sitting on just an intake kinda hesitant to add an exhaust ontop of it because from what I've seen it can actually decrease power

So do you guys feel anything over just intake and stock exhaust?