Welcome to Just Commodores, a site specifically designed for all people who share the same passion as yourself.
Have you tried to give the tranny a service first? =|
And checked the error codes?
A blocked cat will reduce performance and increase fuel consumption. The sound will also change to a "ffffttttttttt" sort of noise. The engine pipes will get really hot too. The easiest way to tell if you ahve a blocked cat is to do a temperature test, if it is hotter at the front of the cat, it is blocked, if it is the same temp front and rear, it is not working or it's hollow, if it is at least 10% hotter at the rear it is working.
Drive the car hard around the block, then while holding the revs up to 2000rpm, test the cat.