Yep. You're right. The Graviton Hellbore doesn't fare very well against the WMG. In addition to the 30% bonus of the WMG, that cliff-like drop-off of damage possibly plays a role as well.
So, let's go back to comparing the PPB to the WMG.
Because of the tech cost difference, I don't think it is fair to assume a ship with the WMG is going to have the speed advantage sufficient to maintain maximum range. So, the PPB devastates the WMG ship. With a speed benefit of 3 to the WMG ship, it seems like the two ships are in a dead heat.