i dropped by my local reliable race shop, and they say HKS and Comptech isn't that loud. As for HP gain, I wouldn't even think about it. Something minor as an exhaust will not make much of a difference. As for Amuse, they make the loudest sound for the S2K. I'm just sharing you some words from what I've heard. Hope that helps a little. BTW, HKS and Comptech is running a little over 1K if you are concern about price.
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