That is because CA is in outer space with many of its demands/laws. He is in PA aka 49 state legality. Punch out the precats, weld in new main... off you go. Retaining a cat is not going to hurt any noticeable power on that fire-breathing 177hp motor. Catted v Non-catted M103s had less than a 10hp flywheel difference in output... 6-7hp I think is the difference WHEN NEW. On a chassis dyno you will not see the difference. BMW also had a catted version of the 8:1 M30 for the older E23s and a non-catted 8:1, power diff was about 8hp crank again.
I agree on the mallet, but it is an easy first test. Hell you can just put your hand by the tailpipe and have someone rev the motor up and feel for high exhaust flow. Plus you can see if it will rev to redline in park... it may not.
Plugged cats also harm 603 Diesels badly, I have repaired two others that were so plugged the vehicle struggled to rev to 3000rpm. This of course is in addition to making nearly no boost pressure. Talk about a slug. Mine was plugged also. Key there is 'was' plugged.
I THINK CIS cars attempt to retain stoich A/F even at WOT. i.e. no power enrichment. You would have to attempt to find a dyno of one with air fuel recorded, but I swear I read this some place. Just another reason I do not like CIS.