Parts arrived on Wednesday and I installed them yesterday. It turned out that the original pipe had broken in two and was being held together only by the heat shield. :o
The car sounded like an old clapped out tractor. Now, all is quiet, just as it should be.
I bought the (supposedly) genuine Volvo pipe but I am pretty underwhelmed by the quality of it. It is nowhere near as good a quality as the original 28 year old part. A few observations:
The new pipe is noticeably lighter than the old one. I didn't weigh them but I'd say the new one is easily a couple of kilograms lighter and likely more.
The welds are pretty shit on the new pipe. I had to grind one down (and probably weakening it) so the nut would sit flush against the flange. Quality control was obviously pretty lacking.
The flange at the catalytic converter end is pretty thin pressed steel whereas on the old pipe it appears to be a solid, roughly 1/2 inch think piece of metal. The flange was bending as I tightened the nuts.
Overall, I doubt it will last 28 years like the original, but at least the car is quiet again.