Re early bred y/birds, If your going to try this system for more than one year (pairing up early December) I think racing your y/birds on the darkness system is a no-no. Come December when you want to pair these y/birds up they'll not have thrown all their flights and I think thats a lot to ask of them. One way round it could be to pair your 2-3-4 year olds up December and let them rear but not your yearlings, let the yearlings rear when your repair in March.
Racing young birds... I dont think there's a better system than darkness.