2000 Pontiac Firebird low engine powwer

It could have, hitting it could have broke the inside of the cat. To test it you could 1 remove the exh and look inside the cat, 2 you could tap on the cat and see if it rattles, or 3 disconnect the exhaust at the manifold and see if it has power, if it does the cat is plugged. Also check for any big dents in the exhaust, when you hit it off you could of put a big dent in it which would cause the exhaust to be restricted. Post back with what you find and will advise.