My best guess is that your cat is beyond plugged and that you have WAY too much back pressure between engine and the cat, I would somehow try to get the cat off and inspect it.

You say that it feels like your engine is running on 5 cylinders, this to me sounds like too much back pressure which is NOT good for your engine especially your valves, i would also do what daniel suggested above, also some good stuff. Best of luck to you.