If you're looking to improve the Corrado's exhaust system, the catalytic converter is the last item to change. Simple fact: With carefully constructed inlets and outlets, the Corrado cat is the best one VW ever made. Buster Smith, my muffler specialist took pressure readings on either side of the stock cat and declared it the best he'd ever tested. If the cat starts rattling (and eventually it will if you use an aftermarket chip that gives too rich a mixture at full throttle), you can either shell out $600 to $700 at VW for a new one (It is illegal to sell or buy a used cat, unless they have been recertified), or search for an aftermarket model.
I had the opportunity to try one by Walker (DynoMax Mufflers) promoted as a high performance cat. I selected the largest one I could use (2 1/4 in inlet and exit). Even that one was not as free as the VW cat, but it was fairly close and cost only $150. Techtonics offers a SportCat, rated for a 5 liter motor (that ought to work on a 1.8) modified to work with the Corrado.
After the cat Buster installed a huge Walker resonator (the longest one that would fit) and 2 1/4 pipe all the way back to the DynoMax muffler using mandrel bends throughout. No question, this really made the G-60 pull harder from 5000 rpm up, but there was a penalty in noise at highway speeds. If you want it quiet stick with the factory setup. No free flowing system will be nearly as quiet. Techtonics offers mandrel bent exhaust systems in stainless steel or aluminized that are both quiet and dyno proven to offer significant performance improvements on the Corrado.