The point of a header is not just less restriction -- a well-tuned header is an active component in both extracting exhaust gases from the combustion chambers and helping to get the intake charge moving. To oversimplify, low pressure left behind one cylinder's exhaust pulse sucks the gases out of the next cylinder, etc. The length, diameter, and arrangement of a header's pipes has a profound effect on how efficiently that works and over what range of rpm it works.

So, there are cosmetic headers that aren't tuned for any of that to happen in a useful range, and there are tuned headers that do a lot for performance...

BTW, I once had a guy try to tell me that there is no point having exhaust pipes that have a larger diameter than the exhaust valves. Does anyone here think having a 1.4-inch exhaust system is optimum, because valves that size are restrictive?