I really, really like that idea of Spear Throws having some synergy with throwing feats (though maybe not cost reductions and everything), it's a really nice idea that would help out throwing builds.
As for the boomerang idea, returning boomerangs and boomerangs that hit things are not the same kind of Boomerang. Returning Boomerangs were used to flush out/scare birds, boomerangs that hit things were designed to fly straight so that they could be flung hard and fast in order to kill birds.