A bird destined for the freezer can only achieve the finishing weight that it was bred for and once that weight is reached it puts the rest of the feed into fat around the lower half of the abdomen.
Bred for meat types such as Ross/Cob will go off thier legs if kept on to long.
Pure breeds such as Light Sussex will very rarely get above 5lb dressed, as the best of the strains that were lost to unselective and exibition breeders years ago.
Crosses between Indian Game and any of the heavier breeds would be better, but still a long way behind a Ross/Cob.
It's not what you feed them, it's how they are bred,so trying to get a silk purse out of a sows ear is a waste of money.

This is the result of a Welsh Black x Welsh Black. Dressed out at 8lb+ on a grain diet free-range.