Yeah, that's the issue...they start to degrade where you don't get a misfire, but the combustion isn't very good. It starts sending a lot particulates into the cat(s) and fouls them. Of course, the cats on the NB are integrated into the mid-pipe, so rather than just changing the cat, a full mid-pipe and cat is needed. Having just installed a new cat/MP on this car, I wanted to make sure it would stick around for a while AND get a little more reliable ignition for what I do with this thing.
I just ran a errand at lunch, the car runs a bit more refined. I noticed the throttle response as being smoother and the power curve feels a little fatter, no really any noticeable increase. I did about 20 miles, street and highway. I was seeing exhaust soot on my bumper before, so I'll monitor it to see if this alleviates some of that.