I had the HKS with rv6 front pipe for a little over 1k miles. It got a very marginal amount louder after 1k, but it was still very quiet overall. Like you said mostly noticeable in the lower rpms. Ended up switching to Tomei type R.
I have it. If you like a louder exhaust its fine. Single exit originally I thought was weird, but it looks fine to me. I had the AWE track and HKS before this one. Volume wise it's about the same as AWE, definitely louder than HKS.
Had this installed yesterday. It randomly starts rattling from the tail section. I brought it back to the shop to make sure everything was tightened down and check clearances. Its not contacting anywhere and everything is tight. The mechanic thought the sound was coming from inside the muffler...