When ypu say, "could not maintain heat set point of 60", have you tried setting them a bit higher, say 65, to see if the actual room temp can stay at 60? With our mini-splits, we have to set the heating set point a few degrees higher than what we want the room temp to be, due in large part (I suspect) to the location of the mini-split temp sensors near the top of the indoor units. You'll use a bit more energy, of course, but I suspect those units would have no trouble heating a building with even moderate insulation to 60 degrees with an outdoor ambient temp of 13. They'll just require more energy to do it.