The cats are the biggest restriction in the stock exhaulst, followed by the stock manifolds. That's why a cat-back by itself does almost nothing, and headers by them selves only do a little.
If you replace the stock h-pipe and cats then headers make much more of a difference, especially as you open up the intake.
Look at Bad Dad's before and after dyno numbers. 307 rwhp. That's a pick up of 32 hp. I would be very surprised if the headers didn't contribute quite a bit to that.
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