Brewing Science and Practice by Briggs, et al, p121, says that the Oberteig (which is what this layer of mud is called) is made of a gel-like material made of about 20% Protein and 40% Carbohydrates and micro-aggregates made of about 4-21% Starch, 3-19% ß-Glucan, 5-31% Pentosan, and 26-42% Protein.
I get significant amounts of Oberteig even when I use a single infusion. I may get more when I step mash (independent of whether I decoct), but I've never tried to quantify it in any way. I may just expect more because I expect time at a higher temperature should result in more precipitation. I do modify my water to correct pH and Calcium levels, which should be a factor, just as it is in the boil. I also tend to get crystal clear beers without significant cold conditioning, often right out of primary or within a couple days of completing carbonation.
I can't find the reference at the moment, but I've read that significantly more protein precipitates in the mash, actually, than in the boil.