I routinely ferment at 46F - no problems and not much lag, as long as a healthy yeast pitch is made. I have not measured the differential in terms of the wort in active fermentation in my fermenter chest, but I would be surprised if it rose to anywhere near 8 degrees. Ales, maybe if in the low 60's, might get that kind of differential, but lagers in the 40's seems a stretch - I could be dead wrong and am willing to be convinced otherwise. Robert, you have a thermowell at the bottom of your Spiedel, so is that measuring such a large temperature differential (ambient to actively fermenting wort being 8 degrees different)?