HFC do make any exhaust louder and test pipes make it even louder. If your in a state that doesn’t care about emissions then go with test pipes if you want the loudest tone. Before I got my Berk exhaust I had the stock Nismo and I didn’t like how quite it was. So I got berk HFC and it really made a difference. It was like my exhaust was sleeping and the HFC woke it up. I personally don’t like very loud exhausts so that is why I didn’t go with a test pipe. But as I said if you want loud then go with a test pipe. Though with a test pipe there is no filter with your exhaust so you will tend to get black soot on the back of your car from your exhaust. I found a 350z with a hks exhaust on youtube
350z hks exhaust & aps test pipes - YouTube but was unable to find one on a 370z. I did find a HFC on a 370z here
PW370Z with HKS and HFC.wmv - YouTube. Though the video is still very different then in person i hope this gives you some idea of how it sounds. Hope this helps.