01+ cars are more sensative to cat converters than 97-00 cars because of the new cat diag inside the PCM.

I would buy a cat converter with a warranty and weld it in yourself if you can. I just did this on my sisters car.

Or price out a whole downpipe unit that "doesnt" run a high flow cat and install it. A highflow unit is what sets the ses off again.

And the non fouler trick just puts the o2 sensor further away from the exhaust gases, it makes it not stick down as far inside the pipe.