You are in uncharted territory with the header design, and I hope it works out.
In the early days of the original SHO, the ShoShop company assembled a tube header system. When the dyno numbers came it, it produced less horsepower than the stock cast iron units.
When you get into thermodynamics, most things are not intuitive; rough surfaces can flow more CFM than smooth surfaces due to boundary layer activity, and it goes on and on. Not intuitive at all. Tube size comes into it and there is such a thing as too large a tube, and it is hard to know what is the correct size is without experimentation. Especially in a turbo situation, which upsets all the standard concepts of scavenging and flow design.
Wishing you the best.