About how they eat the shell and all? The rocks in their crops grind it all up and it get's nice and digested. As for equilvancy to corn, not sure about it but I think it's better with what is in it, but more expensive.
Best of luck, like dual purpose chickens, its hard to "fatten" up birds so to say that weren't really programed to get fat and put on weight. The corn would add a bit of fat to the meat though for a juicier roasted turkey. Not too much corn though since you want to make sure they have enough...