When I flew my youngsters on the so called natural system I found the 2nd & 3rd rounders made the better old birds. This could've been down to training the 1st rounders too late or they were bred whilst daylight hours was shorter than the 2nd & 3rd rounders, or a combination of both. My preference is racing up to 300 mile so not really qualified to answer your question on how to treat distance babies. What I would do though is race them on the natural system as youngsters as I see no point in putting them on the darkness, they'll not be able to compete with sprint based pigeons whatever system used. Re darkness, yes it does take a lot out of babies but if managed correctly they'll perform just as good as natural youngsters do as old birds.