Only real issue I see with whole grains, especially if avoiding soy, is getting the protein level high enough. Broilers, and I'm assuming you're raising cornish-cross here, are bred to grow really fast, and they're ALWAYS hungry. They will stuff their crops at every opportunity, especially if you're restricting feed to try to slow their growth. My suggestion is that if you need/want a longer grow out would be to get a different breed.
Freedom Rangers grow to a really good size in 10-11 weeks, so going to 12 wouldn't be a big stretch for them.
Regardless of what you do, if you're growing them out longer watch them carefully as they get closer to the slaughter day. Any that just don't look like they're as active or healthy as the rest
should be processed by you right away. I raised 113 Freedom Rangers. Only lost 3 to mortality. 2 weren't looking good half a week to a week before slaughter so I processed them as soon as I noticed it. As close as they were to target slaughter day I wasn't about to chance them dying on me and being unsalvageable.