10k stars does not give you more damage, only more attacks and it does also not give you a penalty to the extra shots but a penalty to the Doubleshot effect if you have any item or the Epic feat that grants it to you.
I cannot recommend taking two ranger levels but that's your choice.
So far i'm not aware of any way to bypass bludgeoning DR with a shuriken but if you use the The Morning Star or a triple positive GS shuriken you will do quite enough bonus damage to maintain good DPS even on undead.