Your hybrids will do fine without the extra heat and light (they may drop off a little in their winter laying), but your Leghorns are not as cold hardy and will likely have a considerable drop off in their winter laying. Still, I agree with TwoCrows. I would not add any heat, but a little extra pre-dawn winter light would be fine (even that will add some heat to the coop). Also, you will probably want to smear some petroleum jelly on your bird's combs on really cold days to prevent frostbite (especially on the Leghorn's combs as they are really susceptible to frostbite).
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