I just replaced my OEM cat on the 960 as one of the pipes started rusting through.
I got a Magnaflow 23954 replacement as it appeared reasonably well built compared to the other stuff out there. The Magnaflow uses single wall tubing but it is mandrel bent as opposed to the crush bends used by the competition.
The cat is much smaller than OE so I don't expect it will match the original in durability (over 13 years).
The front O2 bung interferes with the heat shield. I had to bend it slightly but I think I'm getting contact on acceleration. The Magnaflow definitely is louder than the Volvo unit, in cruise as well as acceleration. If I had the $$$ I'd definitely pick the OE Volvo over the Magnaflow.
Anyway, here are a few pictures for those interested:
Cheers,
Richard