A clogged cat will inhibit performance, as well as a bad or stock muffler. The oem muffler is designed to be quiet, not for performance. A budy of mine with a carb'd '84 punched a whole in his cat(could easilly afford to have a shop put on another one but he is cheap) when it was clogged, which did help some, he also ran a header and highflow muffler. Since we live in state with mandatory smog inspections every 2 yrs, when he put a new cat on, his performance actually improved, that little bit of added back pressure really helped. Standard 22r's don't benefit from a completely open exhaust system, unless you are always running above 4k, which is hard to do unless you are racing, they simply don't produce enough power.