I know that all SIP's are different, but in our system the splines simply act as a friction-fitting tool that maintains the thermal consistency between two panels. The Polycore panels are not a sheeted SIP, but an embedded SIP, meaning that the structural steel is embedded within the panel. For standard sheeted SIP's you may want to consider the most insulated material available. If you are simply using 2x splines you are effectively creating a thermal bridge at every seam. Anything you can do to minimize the amount of thermal bridging would be beneficial for the overall energy efficiency of the structure.