The last two GSDs I had were outside dogs while I was at work. They probably were out for 8 hours and seemed to have no problem, granted they were coats and well acclimated to the winters in MN. Even though I had a doghouse built inside the garage with access to the kennel, both dogs preferred to stay out in the kennel most all the time. I'm guessing it was due to the south exposure and the sunlight warmed up their dark coats plus all the "action" is closer to their nose and ears while outside. I do recall on some especially nasty cold days I would leave them in the house and check on them at lunch....same procedure for both dogs, which I owned at separate times. Once, I discovered they were fine in the house, I allowed them full range of the house on really harsh cold days....seemed to be the best solution since they minded their manners.
I remember asking the breeder about my concerns with colder than average weather and she just chuckled at me saying....."....look at their coats...what do you think..." I guess that means I was being paranoid...oh well.
My current GSD is spoiled and pretty much stays indoors all the time while I am away hence she has not acclimated to MN winters as the previous two. When the regular temperature ( no windchill included ) gets to about 0 or so, I notice she will pick up a pad or two off the ground as the freezing temps annoy her. This usually is my signal that it's too cold for her to be on her feet, so if she chooses to stay outside, she normally lays down for a bit until the pads warm up.
It sounds like you have your dogs pretty well adjusted to being outside with their romps all day long on your property while you are away.
I think the double coat of a GSD benefits it greatly in winter weather, especially when it has been accustom to the outdoors over the seasons.