Tormentor buff is additive and not multiplied (for me it was giving 31% in the end), after testing for a bit I came to the conclusion that casting it has the same damage as elemental weapon cast (after including orbs and status effect procs) under a 100% uptime daedric prey. Which means casting it makes rotation harder, deals same damage, but hits your sustain.
Essentially a prebuff and never touch skill.