I have the Corsa Track on mine and it is a good step toward reducing interior temps, but the cats are a major source of heat as Jon pointed out. I find that directing the A/C controls to the lower outlet (leg/foot) is a good idea on this car since the floor area is the prime source of the heat entering (with the top up). Also keep that A/C fully charged! It would be interesting to experiment with some of the space age heat blocking materials now commonly available-- add the material above the cats and under the carpets themselves. Some of the vendors here sell improved insulation wraps for the sills, might be a good idea to investigate/install while you are changing the exhaust.
The stock crossover pipes throw additional heat into the cockpit floor from behind the seats, so removing the crossover is a common step to reduce heat and weight, and all but one aftermarket exhaust for the Gen III removes the crossover setup completely. I believe B&B still utilizes a crossover, which has the advantage of combining sound from both cylinder banks out both sidepipes (you hear 10 cylinders out each side instead of just five).
