E93 catalytic converters

The warning light on 2010 M3 came on saying “excess emissions” and have engine checked soon. I took it in and both catalytic converters registered over .9 on the emissions test. The dealer doesn’t work on them, but instead replaces them - for the modest cost of $7000 (not a typo). The service advisor said to have a muffler shop take a look at them as they can usually repair them. I’ve not noticed any effect on the motor’s performance. Two questions, are replacements really that expensive? And can they really be cleaned out so they’ll function?