Someone know about exhaust performance gain?

To keep it very short and simple, you are correct.

Header design is the most crucial when it comes to a performance tuned exhaust.

That's not to say that from the collector back isn't important, it still is. But only in the sense of controlling volume and velocity.

 

Well I'm not completely sure but I should be... undergrad in Aero/Mech engineering.

Exhaust back pressure can influence the torque and speed of the engine. Headers typically allow for better scavenging of the combustion chamber (cylinder) meaning that if all the tubes have the same effective length and equal pressure drop they will actually clean out the combustion chamber by causing a lower pressure in the header. If you are building for speed this is a very good thing. Sometimes you don't want that such as when you are attempting to increase torque more than speed you may actually want to have more back pressure in the exhaust.