Hi,
It looks from the photos like you have melted the cat. This has never happened to me, but I know of a couple of people who have done that. It is due to the cat not being able to cope with the huge heat produced. Once this starts to happens, it gets worse, as a huge back pressure can build up, producing more heat, and putting more stress on the manifold. Maybe this is why the flex or manifold has blown out??? It's a bit hard to tell from the pics exactly where the leak is.
What mods are you running on the engine eg pulley reduction and ecu? From the limited number of cases I have heard about some set-ups seem to have more problems with excessive heat, causing problems with cats and exhausts than others. It's like oil temperatures, some pulley and ECU combos seem to run incredibly hot, and others dont
I now run with a straight through pipe, which removes this problem. If your car is road registered and you want to run a cat, look at getting the highest flow sports cat with the lowest cell density you can legally run
Early on in the days of racing my mini I cracked a couple of headers (manifolds) that needed replacing. The problem wasn't really with the manifolds, but with the engine mounts. When racing the engine was flexing so much on the mounts that it was flexing the manifold. With the extreme heat of the manifold, and repeated stress, it kept cracking. I fixed the engine mounts, and everything was fine for a long time. After around another 15,000km of racing and rallying I finally cracked another one, but I guess that is not surprising after such a long time
As I said, this wasn't really the fault of the manifold, the first 2 were actually from different manufacturers. It was more the stress placed upon them due to the excessive engine movement. It would be worthwhile checking your engine mounts, and seeing how much the engine is flexing under load.
BTW in the early days I also burnt out several exhausts as well
Cheers
Robbo