(R32 GTR, 330awkw)
I have:
HKS Exhaust Manifold
HKS Front/Dumps
DoLuck Cat + Cat-back with a 6" Canon
I got it tested last week and it was 120db at 3,500rpm. Not only was it extremely loud at idle (have Tomei Poncams too) but the drone was ripping my ears apart (especially when i'd done 4x 250km highway drives in the first 2wks).
I got a local exhaust workshop to replace the rear DoLuck canon (straight through pipe with baffling around the outside of the pipe) with 1 of their own. Their was a flange (4 bolt grr) right infront of the rear canon so it was just a matter of adapting the custom muffler to a 4 bolt flange and swapping it over (also had the entire cat-back raised 3-4cm). The result was 98db at 3,500rpm and absolutely no drone at all which is still way over the limit but i'm not sure whatelse could be done without making the car undrivable.
The annoying thing about this custom rear muffler was that its taken away the awesome "worked RB26" sound and also my power/boost is way down. I changed the rear custom muffler back to the DoLuck canon yesterday in the garage, pain in the ass 20min job. No point going to the drags with half your power stuck in the rear muffler.
I'd really love to get a whole new cat-back that is 1) 90db or less and 2) capable of flowing 330awkw but unfortunately I don't think it is possible. Having a 4 bolt flange on the entire system makes it very difficult to just swap over individual parts with other brands/models to see the difference as basically every but needs to be custom welded.