I don't claim this as fact, just something to think about:
most people would agree when getting exhaust out, faster is better. But it is important to get it out smoothly. Imagine the explosion happening in a cylinder and the exhuast valve opening. No backpressure would let the exhaust leave nice and quick, but right after a vacuum would form. This vacuum might kind of suck some of your exhuast you just let out back in, or at least disrupt the flow leaving the exhuast, and cause turbulence inside the cylinder disrupting your air/fuel mixture coming in. And at lower rpms, I could imagine this is where the problem is more likely to occur because it has more time to build a fat vacuum condition. A little backpressure prevents exhuaust from leaving too quickly so a vacuum doesn't form, at least not much different than what chevy expected so the flow is nice and smooth. I'm sure you would be happy with a high flow cat, how much of a difference they make is another story, but they are a lot cheaper than the ones you would buy from chevy.