Here's a good read on exhaust systems & performance:
http://www.boyleworks.com/ta400/psp/exhaust2.html
As for the Flowmaster exhaust, I'm not a fan at all, but that's what I put on our Camaro, becuase no one else makes a 2.5" mandrel bent system, for exhaust manifolds, that uses a transverse muffler. Pete fabbed his up from scratch with the help of a local mandrel bender for his Pure Stock Camaro, but I used the majority of a Flowmaster kit and some mandrel bends on ours...but, the friggin' exhaust place STRAIGHTENED my mandrel bends then rebent them to fit! Naturally, they busted some of it, and what they didn't bust looks like crap...