Hi jrnymn,
While it is true that tracking and tilting both improve output, the sunlight is still filtered by lots of extra atmosphere in the winter. (thinking of the song "Summer Time and the Living is Easy")
Here is a quote:
"For instance at a latitude of about 50°N solar radiation is spread over an area 35% larger than it is at the equator, so the potential receipt at a given unit of time will decrease accordingly. In addition, the solar radiation will also travel through a greater width of atmosphere and that too will decrease the amount of solar radiation, even at midday."
Variation in light intensity