As for the additional damage type however, ironically enough in the core rules damage type is determined by the weapon, not the ammunition. Check the SRD, the bow is listed as piercing damage, while the arrow has - in that spot. Seriously. It might be something worth changing, but seems like a needless complication when the same thing could be gotten across either way. (though some customization options exclusive to masterwork ammunition may be in order, I'd have to be careful to avoid stepping on the toes of what the weapon itself can do)
>.> I never noticed that. The SRD strikes again. But, yeah, if that's how it already is, no need to create more rules. I like how usable out-of-the box this is. I see a lot of homebrew projects that I like in theory, but are so complex/personalized that it seems like a lot of work to make them usable.