We tried sunflower, flax and white German millet seed in the past. I didn't see any difference in the performance of the birds. We gave it at least a year's trial before quit adding it to their ration. The major difference is the cost of feed. If I remember correctly the flax was almost $50/50 lbs., white German millet $20/50 lbs. and sunflower seed $25/40 lbs.
We also took corn chops out of the birds ration. Corn can and will cause purple on black birds. We now feed, whole wheat, whole oats, soybean meal, fish meal, Vita-Ferm-Cattlemans blend, corn oil, commercial layer mesh/crumbles, grower grit and oyster shell. We try to raise watermelons and tomatoes to give the birds as treats. This year has been very adverse growing conditions. Looks like the watermelon isn't going to preform.