So there I was, barrelling down the highway at about 80 with a bunch of other traffic and out of nowhere appeared a cement brick/rock. It looked like a chunk of sidewalk or something. Anyway, couldn't avoid it and the car in front of me put it in a position to squarely knock the hell out of my underbody. It sounded like it did more damage than it actually did. There is a dent and bad scrape in the cat, but I do not know if it is just the shielding to the cat or the cat itself. Should I be worried? No CEL or loss of power (yet), but I wanted some opinions.

Make sure you get it up on a lift and check it thoroughly. You should be ok. I did a similar thing many years ago - ran over a truck transmission in my 85 Sentra. Put a hell of a dent in the exhaust, but that was it.