dps wise though powershot is 2100 vs flameburst 1300 damage, so the enemy would need to have around 50% damage reduction to make blameburst deal more damage and even more if it is a marauder.
No. You are ignoring the damage from CGC, which is almost 2x your 800 damage differential AND is elemental just like FB. So net, FB is going to more damage than PS, even if the target has zero armor. Once you factor in the target's armor, FB doing 50-100% more damage than PS is common.
Bottom line, there is never any reason to bring a Merc dps into a ranked wz over a similarly skilled player with a different subclass. NEVER. EVER. Merc dps is the worst subclass in the game. As long as your remember that, you won't run into any trouble when selecting a ranked wz team composition.