From what I know, void knight equipments will give additional 15% accuracy and 15% damage of the style of combat. But this is given that you wear void top, bottom and gloves (or deflector). Full slayer helm provides additional 16.67% ranged boost, or attack and strength boost, or magic damage boost when you're on a slayer task. I'm guessing the increase in damage is scaled diredctly through the increase in boost.