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HKS exhaust gives a nice exotic sound, HFC you may have more concerns when it comes to engine matienance and having stuff covered under warranty. HFC is a dealers eyes is a big no no. Some are cooler than others but as a whole they prefer a muffler or cat back system to HFC anyday.
No issues with my dealer and the HFC's they even replace 2 02 sensors for free under warrenty for me
sounds like you want hfc's. test pipes can tend to be a bit raspy although you may see more gains because of less restriction.
also, the g exhaust is 3 sections - y-pipe, mid pipe and mufflers. most exhaust systems are catback so they replace all 3. some are just the rear mufflers though. you can also buy aftermarket y-pipes.
HKS sounds exotic??!! lol thats a new one
IMO (after hearing Greddy, Borla, FI CF and Magnaflow on the G37) FI sounds the most exotic and looks the meanest!