Have someone stand on the gas pedal (in neutral) while you are standing by the exhaust. If there is a restriction the exhaust will sound like it is wheezing. If it is restricted it is not necessarily the cat, some of the older GM stuff used a double wall exhaust pipe. I have seen more than one collapse the liner and it will not show on the outside. It may piss off your neighbors but you can disconnect the pipes at the manifold connection and start it up. If the throttle response returns you are on the search for the restriction.