all I can say is that I did aftermarket in my '04 5.3. My muffler guy, who is old, cranky, and does mostly custom powerboat headers and muscle cars, says anyone that pays for factory catalytic converters is an idiot...
Now let me also say, he said I'd be an idiot to do both at the same time, only one of them was bad. He'd charge me double to do both, no difference in time for him. So he did one, and said next year when the other goes bad, come back - and I did, it was the same price - about $160 if I remember correctly. Never had a problem with them