There are different types of MGUS. They don't all progress to MM. For instance, IgM MGUS rarely develops in to MM but more often into WM. I've seen papers claiming the chances are between 0.5% to 1.5% /year. There is also a stage where it is determined to be Smoldering MM or WM. I've seen studies showing that having a low BMI is better than high BMI at slowing progression.