Tony is right, his system is very nice and might be less expensive than the HKS. Two of my friends have the F.I. systems on their Z's and they love them. Both of them, however, have bottomed them out on the roads here in Alberta and have scratches and dents. It does sit lower on the car than the HKS. As for looks and sound though, they are killer! Not a LOT more aggressive than the HKS, but different. And I like the tips on the F.I. WAY better!
Having said that, my system with the BERK HFC's and the HKS is very efficient and I have free'd up a tonne of horsepower. I will have to dyno it this year to find out how much more. With an HFC and the exhaust, I still think my sound is less aggressive under full throttle than the F.I., yet still has amazing tone and depth in normal driving. No rasp and no drone. Sorry Tony!, but the drone of the F.I. (with HFC's or test pipes) at highway speeds is too much for me to take. I have ridden in my friends Z and the difference is HUGE.
See this thread:
http://www.the370z.com/intake-exhaust/22791-fancy-tech-how-get-rid-exhaust-drone-works-great.html