I'm not actually sure what point you were trying to make in that post.
Backpressure is NEVER desirable or something an engine "needs." The article Mudder linked to explains very well how pipe diameters affect flow velocity but this sentence sums up the idea succinctly:
While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you also want make sure the pipe is wide enough so that there is as little back pressure as possible while maintaining suitable exhaust gas velocity.
Backpressure may be an inevitable byproduct of a system designed to achieve a certain goal but that doesn't make it desirable. As with anything in internal combustion engine design, there will be compromises. There is no perfect engine that can run at 100% VE throughout it's RPM range.
So, the best we can do is learn the basics and that can give us good guidance on how to evaluate what the aftermarket has to offer and whether it will meet our particular goals.
One need not be a physicist or have a book full of equations to understand that smaller, longer pipes enhance low speed torque at the expense of high RPM capability and likewise larger, shorter pipes work better for high RPM engines at the expense of low speed power.
If one wanted to design a custom header specifically for their engine then I'm sure there would be some serious reading and computations involved but the vast majority of people are going to go with what the aftermarket has to offer so understanding some basic concepts is all that's necessary to make a semi-educated choice among the available offerings.
