dave
the strapping was secured with long panel screws and bears directly on the foam.

check this out
http://www.htf.ab.ca/george/pdfresources/wall-roof-detail-3d.pdf

i concur with your thoughts on built up versus panels. two people can relatively easily put this all together with ladders and scaffold, with no need for crane, though if it's there for the raising why not keep it for the panels.

and 2x6 sure seems strong enough that you don't need the structural part of the SIP. and i guess you can screw strapping to it pretty much wherever you want rather than trying to hit rafters or putting down horizontal strapping over vertical strapping on a purlin roof.