DR_ is pretty much correct. HFC's are much cheaper and easier to install. They create a little bit of a raspy sound though so keep that in mind.
I have fast intentions long tube headers which delete the cats and I love them. They sound amazing. By themselves they give you about 19hp, but I have full bolt ons so you have to remember the law of diminishing returns when using more than one mod. Cats vary but you could expect 10-15hp from high flow cats.
My only complaint was that my headers were insanely difficult to install (I did it myself), but that was partly because half the bolts in my engine bay were stripped, and one of my O2 sensors was cross threaded from the factory. Major PITA.
You can check out my build thread (project mistress) for videos of the headers and sound clips of what you can expect
hope that helps, and welcome to premium!
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