The following enhancements do nothing for Shuriken:
-Ninja Spy Capstone: Critical Threat Range increase of +2 for Shuriken (does not change threat range at all). It's called Ninja Master
-Halfing Racial Enhancement: Master Thrower also, like Ninja Master, does nothing. It's supposed to give a +1 Crit Threat increase.
-Kensai Keen Edge, like the halfling enhancement, is meant to give a +1 Competence bonus to threat range, which works with every single weapon in the game except shuriken, despite being advertised as doing in it's description.
Please let us know if you're aware, and/or will address it any time in the reasonable future. Making builds and committing to them is very difficult without this sort of information.