If you want a quiet system the Milltek SS cat back is probably the one to go for.
A GTC titan will certainally not be quiet! And i'm not sure about the HKS system.
Most of the noise from the exhaust comes from the y-pipe. Is titanium setup at the moment resonated or non-resonated?
Sold: White R35 2010 Black Edition
Current: R32 - Nur engined - KH2 - TE37s