I also have a conspiracy theory that crit threat multiplier is busted on the Shuriken, or at least based on testing by people like gibbons regarding how crit threat range stacks bizarrley, that maybe crit multi stacks fucked up, and so they haven't touched it. Remember, right now, the crit threat stacking bug isn't so bad because there there are so few ways to decrease your crit threat range outside of Critzillas bizarre mix of classes and abilities, which deny the insanely powerful use of shurikens with stacking extra procs.
And it seems, the issue with crit threat stacking is not with IC Ranged, as all the IC feats seems to stack correctly. It with non-feat based sources of Crit Threat range increase which really bork things up and double stack, like Crit Rage, Exploit Weakness, Keen, and expanded threat range thrown weapons. So the issue sort of circumscribed, like a zit around an infected sebacious gland, containing it.
However, I theorize that non-Feat based sources of Crit threat for throwers also stack inappropriately, and therefore opening that up to SWB and Bard +Barb would be something of a Donkey Show.
Anyway, insane ramblings of a guy who likes throwing builds.
Post test edit:
Neither Lethality nor Master Builder seem to apply to throwing daggers. Would be nice if someone could alert the Known Issues list on motherboards.
Edit:
Lethality does seem to be applying to throwing daggers, while Expert Builder does not appear to do anything. Unconfirmed reports from Forest that none of the enhancements grant crit multiplier