You can unbolt your cat and look into the flanges to see if it's clogged. I bet it's your muffler, though.

Chop the muffler off. It might not be the cat that's clogged up. As long as you still have the catalytic converter in the mix, it's really not all that loud. I ran around like that for a long time. A friend of mine's LS integra has been like this for 3 years now, and he's never been pulled over for it. Without the cat, you well get pulled over every 5 feet because it's deafening.

My civic doesn't accelerate in 5th gear from 55 mph either, and I have a '95 dx engine. It didn't do it even with a straight exhaust.