Google Sorcerer Arithmagic for the calculations. I think he did a stamina version, but it is easy enough to look at the Sorcerer equations and plug in stamina values (it all works the same even if the resource pool and focus are different)

For calculating how much extra damage 400 WD will add, use the calculator at: https://esoitem.uesp.net/viewSkills.php

Input your existing stats (as shown on your character sheet, after modifiers such as agility, minor/major brutality, etc.) and check the tooltip on an ability, Lethal for instance. Add another 400WD*your WD modifier and calculate how many extra % damage the new tooltip shows relative to the initial one.

For calculating the effect of the added 3450 pen from Spriggan's, use the calculator I linked in my sig.

My gut feeling is that the Sprigg will add more. It also buffs non-abilty sources of damage like glyphs, proc sets, etc. But I'd recommend you to investigate it either way.