First off Welcome!
As far as aftermarket mods are concerned I am all about them if they are proven to be better than what is offered by the factory. In alot of cases aftermarket companie's like Borla manufacture the over the counter GM High Performance exhausts for the 2010 2011 Camaros.
Now to the Z28, I am going to see how choked the new Z28 is when it is released. My good friend Jim has a 2009 CTS-V and he is putting down 578HP at the rear wheels with some tuning and some quality aftermaket parts. The video is on you tube under HPJIM.
I own one 198 Nassau Blue FRC Corvette's and I sent it to Kateck. My car is putting 468HP to the rear wheels! Building a car is all in what you are looking to get out of it! I will tell you one thing. When I get my new Z28 and if it has the LSA motor it will have the following brands installed on my car:
Kook's Long Tube Headers
Donaldson Balckwing Air Induction
Corsa Exhaust
Hurst Shifter