I traded one of my BBv2's for them. The foam rings are about as thick as a sheet of card-stock, or maybe like 2-3 sheets of cheap printer paper. The magic is the purple sliders though, they have a slightly different height that allows the foam rings to quiet the upstroke without reducing key travel at all. You can find some pictures of the differences between the silent purple sliders and the normal black sliders here 
They're just a hair taller, most of the space to accommodate for the ring actually comes from the thinner base. So someone could technically turn stock sliders into similar silent sliders by sanding(?) down the base, but consistency would be an issue and I imagine that'd take an insane amount of time to do.
Another thing to note is that both sides of the base also have cuts in them (as apposed to a single side on the stock slider) to ensure air doesn't trap the ring above the base. This is possibly what creates the "whistle" that people hear on their Type-S (and why it's so inconsistent), it's a ring that slid up the slider on the way down of the key press (they don't stay in place very well), then was pushed back into the base as the key reached its peak.
