Catalytic converters have gotten a lot of bad press over the years, and they've earned most of it. Back at the dawn of the emissions system, lead-pellet catalytic converters were the next best thing to sticking a banana in your exhaust pipe, and were guaranteed to kill power production. These days, though, the better ones have the capacity to clean even a boost-enhanced Mustang's exhaust.
A catalytic converter works as kind of a chemical furnace. In most catalytic converters, there are two stages: one tears the extra oxygen atoms off of the highly toxic NO, NO2 and NO3 -- collectively known as NOx -- gasses, leaving harmless nitrogen gass and free-floating oxygen gas. This oxygen gas combines with unburned fuel in the second stage of the converter. as you might expect, combining pure oxygen with unburned fuel creates loads of heat, just as it would in your combustion chamber. This heat sustains both processes, and the cycle goes on.
Catalytic converters don't affect engine function of supercharged engines the way they do turbocharged ones. On a turbocharged engine, exhaust gases build up pressure on one side of the turbine blades, spin through the housing and shoot out. In this case, any restriction in the system is going to slow boost response and increase boost lag. A supercharger, being belt-driven from the crankshaft, faces no such problems: as far as the exhaust system is concerned, the engine is exactly the same, except for an effective increase in displacement. So, the same exhaust system basics apply to a supercharged engine as a naturally aspirated one.
Old-school wisdom has it that anything you put in an exhaust system is going to decrease horsepower, but increase torque because of an increase in backpressure. First, the "increase in backpressure" thing is a myth. After about 1 inch past the exhaust port, backpressure is always bad. However, exhaust gas velocity is good. The reason old-schoolers saw an increase in torque occur with an increase in backpressure is that the smaller exhaust tubes needed to maintain velocity also happened to increase backpressure. So, free-flowing exhaust in itself is always preferable.
A supercharger shoves more air into the engine, allowing it to burn more fuel. That produces more exhaust gases, which means that you need higher flowing exhaust just to maintain balance. There are a couple of ways to increase exhaust flow. One is to use a cam with more exhaust timing. Another is to have the exhaust ports ported. Another is to use larger diameter header primaries. The third is to use equal-length headers that encourange "scavenging." Scavenging occurs when a "plug" of exhaust gas from one cylinder's header tube shoots into the merge collector, creating a vacuum that pulls the gases out of the next cylinder. Scavenging only occurs within a certain rpm range; but when it does, it acts as sort of a "backward supercharger."
Shorter header tubes with a larger diameter encourage scavenging at high rpm levels, and long, thinner tubes encourage scavenging at low rpm. On a supercharged engine, you're going to want to pick a set of headers that scavenge within the supercharger's operational range. The vacuum of the scavenging will offset the extra boost in the intake, keeping things balanced. In practical terms, that means using longer and thinner header primaries with a Roots-, Whipple- or Screw-Type superchager, which makes boost at low rpm. If you're using a centrifugal supercharger like a Paxton Novi, you'll want to go with headers biased toward high rpm.
If you've optimized the rest of your exhaust system, then a set of high-flow aftermarket cats sized to your header collector diameter will have almost no impact on the engine's power output. In a well-designed exhaust system, all of the hard work's been done within the header collector itself. After that, the exhaust is flowing straight and fast, and the catalytic converter will have little to no impact on performance. Bassani's ceramic-core, high-performance converters are as close to industry standard as you're likely to see; a pair of 2.5-inchers bolted to a 600-wheel-horsepower, supercharged Mustang resulted in single-digit power losses at high rpm. At 600 horsepower, you're not going to notice a loss that low, and the losses would be lower still with larger cats or lower power levels.
