A simple way to check the 'bad cat theory' is to use an IR (infrared) T-meter in a warmed-up engine and probe the temperature into the cat (Tin) and the temperature out of the cat (Tout). If (Tout - Tin) is ~ 40-110 F, then your cat is most likely fine. If (Tout - Tin) is < 10 F or even negative (that is, Tout is lower than Tin), then the bad cat theory is a good candidate for your car.

For the record, at least once before one of our Chicago Volvo dealers declared my 1995 850 Turbo sedan's cat practically on its last legs, ... more than 10 years ago, and my 850 after >215k miles and 23+ years in my hands still seems to go fine with its original cat. In that case, an indie mechanic used an IR T-meter and stated that the cat of our 850 was just fine! This test/check took just a few minutes.