[jmhobbs]

I ran cornish cross meat birds twice, 20 birds each time, and by butchering at ~8 weeks the roos didn't get around real well anymore, given the weight to leg ratio.

Those were pasture raised, with restricted supplemental feed, and just the ordinary breeds you can get at Tractor Supply. I wouldn't be shocked if you let a "real" production hybrid go past butchering age and feed them grains the whole time, you'd probably get some birds unable to move under their own weight.

After two rounds, I gave up on them and now run less aggressive weight gainers that forage better, like freedom rangers.

Obviously just anecdotal, but don't dismiss it outright.