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A lot depends on what breeds you are feeding,
Most hatchery and soft feather breeds do very poorly on a hard whole grain diet and should be feed a be feed a "soft" or ground food diet.
Hard feather breeds like large fowl Gamefowl, and Oriental fowl do very well with a more "hard"/ whole diet.
My birds 
(not including the gamefowl) are feed a set amount of mash feed and are not full fed and do very well on that diet, when they were on a "hard"/ whole diet there feathers and over all condition was poor. I have found that a max of 30% "hard"/ whole diet to 70% "soft" or ground diet work about best on most breeds 
(not including the gamefowl).
Chris
I think you just have to introduce the whole grains slowly so that their gizzards are able to adapt. I raised my last batch of Cornish on a diet high in whole grains and sprouted grains starting from day one. They have the largest gizzards I've ever seen! I'm sure those muscles need to develop in order to process grains efficiently. I'd be willing to bet most chickens could be switched to a whole grain and sprouted grain diet, given time to adapt.
Sprouted grains isn't really a "whole grain" it is a Possessed Grain just like Brewers grains or Distillers grains and either is easier to possess than the same grain in its dry form.
As for the gizzard muscle need to develop, I don't buy it. Gizzard only grinds the food and it does not extract the nutrients.  
Also your talking about Cornish which should be a hard feather fowl. 
		
		
	
	
Somewhere on line there is a study that a collage did on poultry feed and there affects on the poultry tested, the easiest form of feed to extract nutrition from was Mash and the hardest was Unprocessed grains. 
     
Chris