I've also never driven a car w/ a cat. that was that bad, at least not that I'm aware of. I wasn't sure what you meant, if the cat. was just no longer doing it's job, emissions wise, it might cause a CEL but shouldn't drastically change the way the car drives - I wouldn't think.
If it's falling apart inside though, and is now blocking exhaust flow, it won't want to rev, no top end. Being an HX though, only someone familiar with the car, like yourself, might be able to tell the difference h: