In the current list, the mortar suffers from a bit of a redundancy problem. Not only does the hellstorm do everything better than the mortar, but the mortar's role itself is a role largely unneeded in an empire army. The mortar excels at damaging weak, plentiful infantry, but this kind of infantry is a foe that the Empire already easily deals with through their flanking knights and detatchments.
In an effort to improve the flexbility and utility of the mortar, I propose this very simple change. Allow the mortar two modes of fire. One mode can represent its current 5" diameter blast. The second mode of fire would be the traditional stone thrower shot, an attack better at dealing with knights, monsters, elite infantry, and cavalry than the regular strength three mortar round.
With this change, (along with a possible price increase) I feel the mortar would be seen as more of an equal partner with the cannon than a subordinate choice.