I have found that there is no substitute for actual sheathing (osb or plywood) covering the entire house. If you need more insulation then make thicker walls. I used to build in a colder area of the country and the codes made you build a 2x6 wall AND put 2" celotex outside of it (shiny polyicocyanurate foam panels) with NO plywood or osb sheathing AT ALL. This was for r value. The only strength in the walls was "let in" metal T bracing/wind bracing. These houses were junk. They shook for lack of strength and the window and siding installation would always crush the foam and create havoc with the int. window trim and cause the ext siding to be wavy wherever it was nailed into a stud. I don't really understand why anyone would need more than a 2x6 ext. wall for insulation. If all other details are done right and the house is tight then it will be VERY energy efficient. It is not worth sacrificing the strength of the structure for ANY reason and I would insist on 1/2" sheathing everywhere. OSB vs. ply: I would probably prefer plywood on sidewalls and its a toss up on subfloors.