I was playing Mission 8 (Epicurus) and used Overcharge on a Minos Cannon very early in to the mission, Wave 2 I believe. The Minos Cannon seemed to be firing much faster than four times normal speed (and may have increased over time?) and the Overcharge effect never went away. It will still there in the final waves and my Minos Cannons (I only had two) accounted for 99% of damage dealt in the mission.
