That's just it, they don't have to outdamage anyone. They're already near the top of the other two pillars, being top of the combat pillar too would make them overtuned. I'm okay if their damage is lower as long as it's competitive.
With that said, the damage a party member adds shouldn't just be limited to their individual output. A 5.5e rogue giving up 3d6 SA to make a target blinded until the end of its next turn is likely adding way more than 3d6 damage to that target from its own party, but calculating that is going to be tricky.