[Boog]

You can do a little diggin and find exhaust system comparisons. There have been dozens of them by different magazines over the years for high perf motors and street driven stockers.
A tuned length header will build more power in a certain rpm band. Main tube length and diameter as well as collector length determine at what rpm the headers are most beneficial. You wouldn't want to run a header tuned for high rpm use puttin around town nor would you want to run a header tuned for low end torque in a high speed race situation.
Headers do give the vehicle a different exhaust note. To some loud pipes make them think they have more power. IMO some smooth ramhorns flow plenty for anything up to 400 hp or so.