1 pt = +10% more damage and + not sure how much accuracy, but accuracy relative to weapon's throwing requirements such as, higher PT than the requirement increases accuracy further.
WPF adds accuracy, speed, tons of accuracy while moving, shoot speed and like other WPFs it adds damage. The amounts? I dunno.
If you want snowflakes to 1/3 of a lamellar guy's life, you'll need 5- 6 PT and I suggest 51 WPF due to the most gain for a low cost.
They'll be pretty damn accurate too. Man, fuck shurikens they are crazily good.