[Steeler]

Not sure on the make up of the cats Josh.
It is a Magnaflow s-pipe and cats sold as 1 unit.
Did not really care what the cell count or materials were after the previous failed attempts, just wanted a completed system that would pass emissions. MF says it is C.A.R.B approved on their website. It is not an exact fit for the 3.0i M54 but some simple fab made it an OEM looking system.
Your suggestion of the SS Y-pipe and can worked out nicely and as you say gives the car the "right" sound.
I have added j-pipes to screw my post cat sensors into but do not know the results yet as far as emissions readiness goes.
Still a bit of trial and error methinks before it is 100% emissions proofed but the sound is there finally.