I have always thought that this is the biggest factor. Since the lake is 1/3 of the size it was in the 1980's, I wonder what lake level the 10% is based on? The lake definitely has a smaller effect the smaller the surface area is.
With a growing population in an already arid area, a 10% loss in average precipitation/snowpack could have a huge effect on the area.
Snowfall has decreased greatly in the last decade in the Salt Lake City area; more so than the surrounding regions. Lower lake levels definitely contributed some of this, but I wonder how much?
One of my professors at the U of U has done some studies on the factors that affect lake effect, I will try and find those papers as they may interest you. I recall that he found that pattern is much more important than lake size, but even so, its clear that the smaller lake means less intense lake effect at the least, and we've seen that the last few years.
I don't know what period the 10% is based on off hand, but I would also be interesting in seeing how much as well.