Why a sixth turret would not improve Moltke cheaply

Would a six-turret Moltke-class battlecruiser be effective?

Not as a simple modification: the extra turret would add damaging weight and cost speed, so Moltke would need a broader redesign instead.

The five-turret Moltke design leaves no easy location for a sixth mounting. A superfiring turret forward is the likely addition, but its barbette, ammunition, and structural support impose substantial weight. That penalty would threaten the speed expected of a battlecruiser while leaving the awkward wing-turret arrangement in place. A comprehensive redesign might help, while simply adding another turret would not.