I just binge read the whole thread! Isn't header design fascinating
Try
Engine Analyzer Pro for an engine simulator that includes the exhaust. The demo is fully functional (and doesn't expire if you never shut the program down) and will run many designs in one go. It also calculates bsfc and shows pressures etc. I've either wasted many hours running sims or learned a great deal playing with this program. You can also specify the intake flow to simulate light load cruise conditions. And I believe it comes with a b series file which should help you approximate potential unknowns like bearing sizes.
(I have no financial interest in the company)
Also of interest are these structures inside the 4-2 merges of the OEM header for the 20v Toyota 4age. I've only found a few pictures of them and read even less explanation/speculation about their function.
It's been awhile since I've read motoman so I can't remember if he mentions the old 16v 4ages went from a large intake port design to a smaller one and improved performance.