After hearing an exhaust leak from underneath the car; examination revealed the Cat had a split weld toward the front of the can.
I figured my options here in CA were:
1) New Cat from the dealer (quoted $1760)
2) Source an aftermarket CARB compliant CAT (didn't seem worth the high price and bad rap that aftermarket cat's get)
3) Source a used Cat from a scrapyard (I got quotes of $250 + $50 core charge). My car already has pretty low mileage (110K for a 2005) so I figured any used part would be susceptible to failure too in the near future.
4) Source a used Cat from craigslist (didn't have much luck)
5) Have a local muffler shop repair my Cat (guy quoted me $400 to take the part off and weld it. Or $20 if I took the part off and handed it to him).

I went with the $20 option. Pics below.
If you're in the San Diego area, I highly recommend John @ Lakeside Muffler and Welding. Not only did he repair the cat; he also spotted (within 30 seconds) the beginnings of another crack at the pre cat and fixed that too.
Despite the extra repair needed to the precat he still only wanted $20. Needless to say I gave him a hefty 100% tip.

Followed the article on PelicanParts for removing the exhaust manifold

Split at the front of the cat

Ready for re-install. Both welds visible.

 

How difficult was it to remove the cat? Do you have to remove the manifold too?

Ian

 

Yes. The manifold and cat are a single piece. Not too difficult. An hour if you’re quick. 2 max. Pelican Parts have a nice write up with pics

 

I am glad you were able to fix it, I have done a few of these, but always cut off a bit of steel off the bell flex to give me better room to weld.
You did a very nice job and thanks for us showing what you did!

 

Mine cracked at the other end (where yours was just starting to crack).

It was actually kind of a PITA to get off, because all the bolts at the bottom were rusted and had to be cut off / drilled out. I probably should have just gotten out the oxy acetylene torch and cut them out, but I want to try and keep the non bolt parts perfect, and I also don't have a proper lift. So I used a carbide tipped blade with an oscillating saw to cut off the heads first. That still didn't let the stud pop out, so I ended up drilling. So I should have just started with drilling. Tedious, but straightforward and pretty reliable.

The header to head bolts, mercifully, came out cleanly. A stripped or sheared bolt back there could quickly turn into a nightmare. Like Ladsjohn, I got new gaskets on both ends, but if I am honest, both could probably have been reused fine (they are nice heavy metal gaskets).

It's a bit of a puzzle to then extract the header and cat assembly (down and out the bottom). It will go, but only one way, and it takes a bit of prying and wriggling. (Disclaimer, our mini was lowered and Dinan kitted before we got it, so I am not sure which bits are stock still, and that makes some things harder on this car than they would be on the average Mini. Not sure if this is one of those things).

One mistake I made was NOT marking a good index mark on the two broken pieces before removing them so I could line them back up for the weld repair. Mine was broken all the way through, not just cracked. So I had to guess what was the right rotation when I had it back on the bench. I guessed really close, and it worked, but it was risky. I didn't want to reinstall the whole mess just to scribe a line.

I started trying to MIG weld it (flux core, no gas). Meh. The shroud around it made it a little awkward to get in there, and flux core MIG is a bad weld on a good day. So I broke out the oxy acetylene torch. That worked lovely, that's good thick steel on that header, and the torch did a great job for putting a nice weld on there.

It went back together pretty quickly and easily, and seems to be solid (after a month+ of driving).