This issue has now been solved
Advanced Training Hall, Tier 3.
Yellow quills do 58 damage.
Green quills do 200.
"Upgrading" to this "ultimate" skill cuts your damage to 1/4 against the target you are actually attacking. Instead of vengeful quills adding some extra damage to some extra random target, the skill randomly assigns a different target to take the brunt of the damage. It ceases to be a reliable skill for damage.
Sounds like a bug. It's still useful for poisoning stuff, though!