Headers typically quiet cars down at idle and lower cruising speeds. Once you go WOT, it is a completely different story.
I went from a Lethal O/R H with Borla S-Types and it was loud at startup and idle. Now with a set of Kooks 1-3/4" LT's and their 3" O/R H-pipe and the S-Types, my car is quieter at startup and idle. However, it is insanely loud at WOT.
2014 GT - Track Pack, Leather Recaros, Electronics Package, Back-up Camera - Ported CJ Manifold, SCJ Mono-Blade TB, Custom Intake, Kooks 1-3/4" LT's, Kooks O/R H-pipe, Borla S-Types, AED Tuned - 443 RWHP / 388 RWTQ
Ordered: 3/19/2014 - Built: 5/13/2014 - Delivered: 5/19/2014