Most broiler/fryer type chickens are culled at 8 weeks, aren't they? 22% is standard for chick feed. Too much calcium OR protein would be bad for layers and roosters alike. 20% as in Flock Raiser is NOT imo too much.
I have a single chick right now with weak legs. If it passes I will get a necropsy done to see in fact IF it is botulism since other than Mareck's that is what came up... as you mention, And I really don't suspect Marecks. I also suspect genetics or possibly internal parasites (testing soon)... but not botulism. If indeed it was botulism I would expect to see more birds ailing (and I would report back to you a confirmation as I do suspect your suspicions have some foundation and I'm all about finding the truth). I wonder how many you're questioning claiming Mareck's actually being botulism were even fermenting? In fact, especially in water fowl, how many people simply don't clean out their "dry food" often enough and it actually gets rotted by the moisture left behind that was just wet and not fermented?
I've been fermenting for a long time... 1.5 years. Never had a an illness or a death, yet (except 1 egg binding). My first bird with symptoms is a chick of unknown mixed Silkie breed genetics, 8 weeks old... and doesn't have diarrhea or act lethargic in any way. Just week legs. The following link says botulism in FF took place when stored in an air tight containers, which is NOT how ANYONE I know of does it.
http://www.raising-happy-chickens.com/fermentation.html
18% does seem like a happy medium if you don't have any chicks or feed extra Scratch. Can you provide your sources about rooster needs please? Even though roosters don't lay eggs with protein in them... don't males in other species also have larger muscles requiring more protein to build them? Muscle builders always make sure they have good quality protein available after they work out. I'm being sincere about having a discussion here.. not argumentative or ugly.

I have roosters and want the best for them.