I would say particlewave is wrong, because there is a difference. All headers are used in the test were headers with tubes that had the same length and volume before they come together.
The stock 986 header is just something else. And it is quite heavy.
But for shure one main aspect is the overall diameter of the exhaust system. And the 986 stock exhaust system is just a joke when it comes to diameters.
So in general a complete exhaust stainless steel system with an overall diameter of 3,5" would be much better i think. Also should only take as much catalytic converter cells as you really need to reach emissions. That combined with a optimized intake and a individual ecu tune will give noticable results.
But some shiny new headers on their own - i don't know.
Regards, Markus