I think what Kevin’s getting at, and what would be most useful to players and the game as a whole, is that the current system is badly flawed, and tossing arbitrary damage bonuses on various things with the mentality “yeah this should do damage if thrown” isn’t going to make this better (though it might be a step in the right direction).
What’s really needed is a reasonably simple set of standards that detail how much damage and range a particular item should have, which can then be applied to various dedicated throwing weapons to generate consistent and reasonable stats. For reference, you might look at the design doc for melee weapons on Github. It details each of the main stats of a melee weapon, and what real world characteristics and features would determine each stat (e.g a proper handle substantially increases the to-hit of a melee weapon, but only being 6 inches long, like a dagger, decreases it). Without some form of standardised guideline, any attempts to “fix” throwing weapons are going to be vague shots in the dark at best.
That said, the only truly effective thrown weapon I can think of is thrown spears or javelins. Knives and tomahawks rarely have the penetration to cause serious damage, and most blunt thrown weapons are essentially equivalent to a strong punch. Shurikens are interesting in that every reasonable source (That “Ninja Truth” video seems extremely Hollywood) I’ve looked at suggests they were a backup or emergency weapon. Either as an easily concealed emergency knife, to distract someone, apply poison, or be dropped on the ground to act as caltrops. Just from looking at them you can tell that it would require massive amounts of power behind them to cause any serious damage, unless you got exceptionally lucky and hit an artery.
That said, I wonder if that would be the correct way to go with thrown weapons. Pathetic base damage means they’re in no danger of becoming overpowered, but high critical damage (If you assume zombies need blood and/or intact brains to function) would make them reasonably useful.