I found there seems to be no benefit to running marathon winter tires at a lower pressure. The sidewalls are quite thick and stiff so running them lower seems to just rob you of power rather than improving ride quality. On my touring bike I usually run them on the high end.
I ran a marathon winter on the back of my mountain bike one season, and I think an Ice Spiker Pro is actually a faster tire due to its thin flexible sidewalls despite being much chunkier.