high flow cat converter or stock???

Since removing my catalytic converter, I've tried every O2 simmulator on the market that I know of (except GS Motorsports, they're probably next) and I can't get rid of the stupid CEL. For some reason, they say 03's are harder too trick. So my question is this.... How restrictive is the stock conveter? 5-10 hp? or more? I've heard some people say that the aftermarket high flow cats are not really any better than stock. Others say they could tell a difference or saw a difference on the dyno. I would like some real world feedback. Which brand is best? Dynatech states their PowerCats are up to 54% more flow than stock and even some of the aftermarket high-flows. I know a lot of GM and import guys like the Random Technology stuff and it seems Magnaflow has become very popular lately. My only concern with Magnaflow is they do not blatantly advertise theirs as high flow or for high performance use on their website. Instead they seem to talk more about the fact they are made by Car Sound. Let me know what you guys think. Thanks.