You get an extra 10 feet per range increment with a composite bow, and, unlike regular longbows, you can use composite longbows while mounted. I don't think you can upgrade them, though, as they talk about being made with a specific Strength rating, and the rules for upgrading items are only for adding magic to them. For an extra 25 gp, it can sometimes be worth it for a composite longbow (especially if you're a mounted archer), but I don't think the extra range alone is worth it for 45 gp with a shortbow, even with its much lower baseline range.