Today the auto makers are trying to get every last ounce of power and efficiency out of their vehicles. This does get tempered slightly by cost of materials and needing sound dampening but the pipes and systems themselves are NOT an issue. Gone are the days of aluminized, narrow, crimp bent tubing with brick style cats and poor flowing mufflers.
Cut the factory muffler and flappers off, get something better flowing, and what little bit of performance improvement there is will be done for cheap.
Paying $600+ for an exhaust system that is 0.25" wider and marginally better flowing than the factory one is silly. $1000 plus for a (maybe higher) grade of stainless steel is ridiculous.