Pact belts sacrifice health for an increase damage in that element. Roughly double most of the time.
Video proof above. The catch is a select few abilities already have bonus damage that the belt is additive to, namely Power Chord at close range < 2M , so the overall final damage isn't as a big of a jump. It is unknown whether the other damage passives are additive or multiplicative to the entire damage formula however.
Outside of Power Chord though, there aren't many abilities that inherently have bonus damage that the belt. The Saarabas however does have one passive that also boosts damage to an element by 125% of the staff's element. Testing needs to be done on how this interacts, using different belts.
I have screenshots of the necromancer using winter's grasp with the inquisitor's weapon and hakkon's weapon too at level 1.
Updated: 125% of the particular element confirmed.