[Unobtainium]

If you doubt the benefits of stepped headers, look at racing headers.
F1 has at least 2 steps. It's more than just gradual flow increases.
There is a whole science with sound waves inside an exhaust system.
The choke points on collectors, merge angles,etc all influence overall performance. It really is a case of you get what you pay for.
Making a header better than the factory manifolds is easy. Making a great header is another story.