in my experience "unclogging" cats, most guys just do the "jam a pipe into them" pick out the big chunks, and call it done.
then said "jammers" complain afterward the car ran better, but it's now back to where it was, or worse.
the problem with doing this on the car [or off for that matter] is you get the big chunks, but not all of the material. what's left will eventually find it's way to the muffler, then clog it up, choking off the exhaust.
the only sure fire way was mentioned above. take the cat off, cut open the top, remove the material, then weld the top back on. that way, you can be sure all the material is gone.
or get a "test pipe" [made or bought] installed.
i have fixed too many "jammer" repairs over the years, and heard them cry about buying new mufflers.
anyone that talks about punching out cats these days, i just tell them to not come back complaining to me when the car starts to run worse than before.