http://www.ecosteel.com/tech/insulatedpanels.pdf
EcoSteel offer a 4" structural insulated panel with R-14, and a 3" insulated panel with R-24. A 10" stack of SIP + IP + IP would have R-62, and a 13" stack would have R-86. As EcoSteel are happy to remind you, these are "whole wall performance" figures, including thermal bridging effects. So a 12" SIP would be a serious performer, perhaps even overkill for any application other than a northern Passivhaus.