There is actually a bug with Molten Weapons and both its morphs using with staffs.
For Ice and Fire Staffs the 40% extra damage are not always added. It does normal damage ~50% of the time.
For Lighning and Restoration Staffs only the last tick gets 40% increased damage (here it works all the time thought)
So for channeled heavy attacks Molten Weapon increases the damage by (40 * (1/ ticks)) ~10-14% instead of 40%
I tested on Dreughs near Wayrest.
here is some data. First Fight is always without Molten Weapons, second with Molten Weapon active. All Attacks were fully charged and i was moving.
Ice Staff

Fire Staff - no idea why it does different damage on normal heavy attacks.

Ligthning Staff

Restoration Staff
Edited by mananull on March 17, 2015 9:03PM