Having literally never used this set ever, is it possible that the boost is to the distance knockback is generated? Even if not, five stacks of that IO bonus would only be a 10% improvement, which would be .067 for a standard knockdown, bringing it to .737 which is STILL less than the 1.0 required to actually knock something somewhere. A 2% boost to your standard .67 knockback would be .0134 which again turns that .67 knockback into a .6834 knock.