Chris (OP)
Having had a few custom exhausts on a few vehicles, I understand drone and how subjective sound is to different people.
On my nismo, first I swapped out the OEM cats for Berk HFCs more really more for performance than sound. Still, I did think the OEM nismo exhaust was way too quiet.
Looking for a simple solution, I installed the TopSpeed axle back. This sounded incredible...many different notes available depending on gear and RPMs with a great burble thrown in. Problem was in the cab just too loud. I could never just relax and drive- it was always loud. Maybe like your FI? This may also been because I had the Berk HFCs?
Read here on the forum about good experience with the CNT Racing cat back. They said zero drone (even with test pipes) and showed pictures after three years which still looked like new. I now have this installed and like it. Even with the HFCs,
NO drone and driving I can make it relatively quiet or get on it and make it loud.
The only thing I'm not crazy about is the hiss or whatever you call it that comes along with having the HFCs. Not a big deal.
Nothing against any other exhaust brands, just my experience, in case it helps. Also, here is link which might be interesting to you:
CNT cat back review w/ video
It looks a lot like the HKS with the helmholz cannisters to take care of drone.
Good luck with your search!