Americana roos can get nice & meaty for you, but they probably won't be ready until at least 16 weeks, more like 18-20. There may be some special feed you could use to coax a few more ounces on them, but I don't know if that's worth the extra cost. I think that specialty broiler booster feed is for those hybrid meat birds designed to grow out fast.
Depending on the current situation in the flock & who I have to feed & house where, I will sometimes keep my standard meaties on chick start & grow feed until they're butchered. Other times, if I need to save the extra $$, I'll give them layer pellets after about 10 weeks or so. Sometimes I'll kick them out of the coop first thing in the morning & let them roam the yard finding things of their own to eat for most of the day.
The night before butchering I put the candidates in a wire rabbit cage with no feed, just water. It helps get their crops & most of their intestines cleared out. That also makes it easy to catch them when it's their turn at the table.
