Planning a shop build, any suggestions?

As long as there are the typical 1x8 boards that support the metal roof the foam will be just fine. I do agree though, 8' with no horizontal support boards would be too much. Over time, loads will bow the roof down and eventually snap the foam. That stuff will stick to anything. I build my homes with closed cell foam in the rafters on the sub straight of most homes. I have yet to see the foam fail. I try to create homes that are insulated to R60 throughout with no thermal breaks. I use I.C.F. on the walls and foundations.