Posting in case it is a bug.

Power Shift reads “Damage is increased by 10% each time damage is dealt. Max 10 stacks.”

Expected functionality: Damage increases by 10% additively for a total of 100% increased damage or 2x multiplier with 10 stacks.

Actual functionality: Damage increases by 10% multiplicatively for a total of 159% increased damage or 2.59x multiplier with 10 stacks. This is a 30% damage increase over expected.

With the new Funerary Pick, the damage difference is greater.

Damage if additive: 200% or 3x multiplier with 10 stacks.

Damage if multiplicative: 519% increased damage or 6.19x multiplier with 10 stacks. This is a 106% damage increase over expected.

If this is a bug and deemed worth fixing, the damage can be moved onto the Funerary Pick itself. ie. 700% increased damage instead of 300%.