I am following these threads on XPS with interest. I'm sure you could write your PhD on XPS modification to get better needed properties for certain circumstances. Its an interesting topic.

What about mixing multiple materials? E.g. xps in certain locations where its properties better match requirements or mixing layers of PVC & XPS i..e. sheets of H80 over an xps core to push up certain properties but reduce cost.

It does seem to obvious to me that cheaper/lower density materials can be applied in various places to save on weight and cost, without sacrificing any critical properties that are needed locally.

I guess the first steps would be to identify what the requirements are based on scantlings? Or FEA?