I fed to the Welp Hatchery guidelines (I think 16 or 18% protein), but I also fermented, and let them free range as long as the sun was up. Mine could fly, hop, run, scratch, dust bathe, and everything until the end. The ding dongs even chased flys. They never knew they were fatties. Went to 8.5 & 9.5 weeks. Now, I definitely couldn't have gone on any longer at that pace, they weighed between 4.5 - 7.8 processed...so I'm guessing thats up to 9.5+lbs live... They had a small bit of fat against the skin of the stomach, but we pulled it all out before weighing & bagging. None around any organs. I've definitely seen more fat on Rangers & layers before...so it seemed like they mostly put on meat. I was very happy with the outcome.
I have videos of them chasing me. They could definitely forage on their own (although the preferred for me to show them where the clover patches were) and dig up bugs.
I can understand wanting to grow them slower, but I was definitely trying to take advantage of their ability to gain, so we could have a few weeks off before vacation and starting pigs, more layers, and turkeys the last week of May...I definitely needed a 1 month break between all the craziness.