Mages/Archmages have a similar ability, and so far, no one has complained. The "box" isn't an option for them; the unit was originally conceived differently.
The idea might not be bad, but then we'd get a second, smaller copy of battle mages.
Even before, improved archers had above-average damage, and now it's just insane.
Besides, the piercing shot was also introduced in Heroes 6, and it was quite effective.
