I use 24% the first few weeks of a chick's life to help with weight gain, feathering, etc before they are moved to the grow out pen outside, and then to join their flock mates about week 6-7. Due to my ducks, 24% is the highest I will feed (as higher protein is associated with increased incidence of "angel wing" in some studies).
I've seen no evidence of damage from such a high protein (as compared to typical) feed, but evidence of benefit (particularly as compared to cost) is pretty marginal. Unfortunately for purposes of this question, I'm in the middle of a "mutt" breed project, so there is simply too many variables to attribute effects, confidently, to the protein content of me feed.
Gut feeling, for what its worth, is "doesn't hurt, probably helps, might not be worth it."
Like you, 18% protein is what I feed my adult flock, and I'm mulling over a change to 20% as many are entering molts of their own right now. My other alternative is weekly culling for the table, and thus far, that's winning out.