We're in the process of changing exhausts on my G37. I have dual GTM 2.5"s with GTM test pipes. For me, it's got just too much drone in the cockpit at low load, 2000 RPM.
I like quiet: I had Sam provide recirculation for the waste gate AND the blow-off valves. Got what I wanted.
That said, although the 2.5" duals perform well, I want it even quieter. We are going to cut the test pipes to add in a stubby resonator and change the exhaust to an ARK 'grip' system. I'm hoping the changes will be dramatic in sound, and not dramatic in power change (unless it's more...
)
Back to cats: I have the stock cats, and when I need to get an emission inspection we can simply replace them, get the test, and return to this setup.
Without question the cats (either HFC or OEM) provide emission compliance. They also provide a restriction. It's your choice. This is mine.
Coop