When the cats get bad enough, they typically cause random multiple misfires, due to back pressure.
One tends not to notice the performance loss due to VANOS seals, as well as failing cats. Do the cat rattle when struck with a rubber mallet?
A back pressure tester can be fitted to the pre-cat bungs.
Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.