No, the dmg is:
40 essence = 400%
300 essence = 400% * (1 + 3% * (300 - 40)) = 3520%
Even more
600 essence = 400% * (1 + 3% * (600 - 40)) = 7120%
That's why mage can run so high.
BTW, I think we should do more devour when land of death is active, for me, I changed to
if...