Unless you have some perfectly straight and flat post faces a wee bit of space for inserting the sheetrock would be dependent on how not-straight/flat your post faces are. The inner face of SIP's tends to be pretty flat. Start with the thickness of your sheetrock and add a minor fraction which will overcome your worst case post face. A plywood nailer down the middle of the post works fine and won't split when attacked by the SIP screws. If you glue this 1" wide or so nailer in place you won't have metal competing for the same space.
