Okay, I did not mean to be a wet blanket.
If you are willing to cut and weld the exhaust system, then an aftermarket cat is a better deal.
Reason I mentioned it was that I once was a passenger in a very old honda civic which had some water in the gas, and the car was misfiring and putting raw gas into the cat.
The car ran poorly, but it ran, and the driver just wanted to get it home. It was only when someone passed us to say we had orange hot and molten things falling from under the car that we found out that the cat casing was melting away and falling on the road. I would have never believed that was possible if I had not seen it myself. When we opened the hood the cat (it is right on the exhaust manifold in that car) was glowing very bright red. It was a sunny day in the winter, and you could see the cat just glowing very plainly.
So now I have a lot more (excessive?) caution about cats.