Combo strike and smite query.

Just wanted to enquire. For combo strike it reads "make a melee attack. Instead of making a normal damage roll, the POW of the damage roll is equal to this model's STR plus twice the POW of this weapon". Does that mean when a model with combo smite hits it hits at 20 total or 20 total WITH a damage roll. Also, combo smite reads "the model hit is slammed d6" directly away from this model. If the model hit has a larger base than the attacking model, it is moved only half the distance rolled. When resolving damage for this attack, the model hit suffers a damage roll with POW equal to the STR of this model plus twice the POW of this weapon. The POW of collateral damage is equal to the STR of this model." Can this damage be boosted? Can a marauder combo smite a collossal and boost to get 4 damage dice? Thanks in advance for help with clarification.