What you fail to realize is that the cat is only part of the exhaust system. Look at the flow difference between the stock cat and a test pipe. Yet when people running a test pipe along with an intake, header and exhaust put their car on a dyno they get very similar results to what they would get with the stock cat. I'm not saying that the stock cat cannot be out-flowed, the point is that given what the rest of the exhaust system is able to flow even with bolt-on mods that the cat doesn't slow anything down. Only when the rest of the exhaust system's flow exceeds the capacity of a non-clogged stock cat (i.e. 2.5" collector, etc) does it become a bottleneck. Also, the "not-so-recent header test" you're referring to was on an ITR which is already putting out 25 hp more than a GSR and benefits more from increasing the exhaust flow.

So yes, if the exhaust system is upgraded to a 2.5" collector, the engine is upgraded to at least ITR specs, or the stock cat gets clogged then by all means a replacement cat is a good idea. But on a stock LS or GSR motor with a bolt-on header and exhaust and relatively new stock cat, replacing the stock cat serves not much purpose.