Catalytic Converters:
The catalytic converter, as it was originally introduced, was a large chunk of exotic metal that would in theory catch unburned fuel and combustion byproducts and break them down to produce cleaner running vehicles. They were also, at conception, MAJOR exhaust system restrictions causing severe loss of power. After many people gutted or removed their cats, the NHTSA stepped in and set a standard that the cat can NOT exceed 10% of total exhaust system backpressure. What does this mean? This means that the catalytic converter today is of minimal flow restriction and will always, when functioning properly, outflow what the engine is capable of producing in terms of airflow. The catalytic converter still functions basically as a honeycomb of exotic metals that, once heated creates a second combustion event burning unburned fuel, breaking down NOX, HC and CO emissions. Removing the cat, aside from being a federal offense, punishable by extreme fines, as well as in some states, impound and siezure of your car, is not going to magically free up horsepower as it would in days past. It will usually hurt performance to remove the cat, as well as severely impact your wallet if caught. There are numerous high flow, aftermarket cats available for very reasonable prices if you require one with larger than stock inlet and outlet diameters, and I would strongly encourage anyone looking for more flow to check them out.

I forgot where I got this from. But test pipe won't get you more than a few hp...