Headers can get rhaspy, and always will be rhaspy but with the right exhaust set-up you can minimize a lot of it. As for a dyno of the gains I do not know but you can instantly feel better throttle response due to the better flow and not having that pre-cat clogging up the system. To run a header though you will need to use the non-fouler trick since you are removing the pre-cat and will get a CEL otherwise,