Hellbore has 250 DPS, Devastator has 581(if you cut range by ~60%) and HAG has 480.
Think of it like a slightly faster harpoon that can't be shot down, not quite as good at finishing off hull as the HAG but it's great at popping holes in ships like the Dominator, Onslaught and Invictus. In fact, come to think of it, there is really nothing an invictus can do to stop a hellbore shot, probably strips it's armor in 7 or 8 shots, and it can punch straight through most fighters.
AI has perfect accuracy and also has trouble hitting things with Hellbore. It's just a slow projectile that even cruisers can at least partially avoid.
Elite Ballistic Mastery. It's basically essential for any long range ballistic build.