A couple of days after a snowstorm here in Colorado, I noticed that my 14 panel (Sunpower 225 W each with Sunpower 3000 W inverter) grid-tied system was only putting out about 250 W when it should have been putting out about 2500 W based on previous experience. I went out to clear the snow off the panels, and discovered that each panel was covered on only the bottom 15% or 20%, the rest having melted. As it melted the snow slid down to collect on the bottom lip of the panel. So 15% or 20% shading was causing a 90% loss of power. This experience reminded me that solar cells are diodes, and they must act like low resistance paths when they are not illuminated, degrading the other cells. My shading was a different geometry than yours, but it makes me think that shading of some of the solar cells may have more than a linear effect in degrading the performance of the overall system.