Quick background, I purchased my 99 WS6 without Cats. Previous owner had put a 2.5" ORY on. I had to put cats on to pass emissions. I purchased 2 x Magnaflow 59956 metallic spun cats ($88.00 ea). Installed them yesterday. To be honest, I wish I'd done this right away when i first bought the car. Adding the cats seems to have eliminated a lot of the drone i was experiencing and my car no longer stinks like an old snowmobile anymore. If there is any loss of performance, i can't tell. I'm super happy with the turnout.
Next for me is to install my 4" Corsa Clones that I bought.