I remember seeing somewhere that it was supposed to be 2.2x but after getting and using it some people, especially the lower levels (due to auxiliary shield) seem to survive the headshot. This leads me to believe that it actually has a 2.0x multiplier.
I found where I saw it. https://forums.daybreakgames.com/ps2/index.php?threads/pc-hotfix-12-22.244091/