high flow cats are THE component to finish off your basic bolt-on package (intake, header, exhaust). Some do the test pipe route because they don't need to test for emissions, however, high flow cats work just as well while not polluting.

I've had both major brands: Random Technologies and Carsound. The Random Tech broke in half just 3 months after install. Immediately, I purchased a Jackson Racing converter (rebadged Carsound) and I've had it for almost 2 years with no problems. Many say Carsound is great, but other companies are starting to come out with their own as well.