I was just thinking about this further and realized this may represent a convenient way of producing a system of weighted flies. The key is the volume of the fly stays the same, but only its density changes to produce a different mass. All other materials on the fly are identical.
I did a quick example based on the quoted densities of tungsten and brass beads, and lead and lead-free wire.
This table represents the relative mass of the 4 combinations:
Lead Lead-Free
Tungsten 1.00 0.85
Brass 0.66 0.51
You can see that a brass bead/lead-free combo has half the mass of the tungsten/lead version. In this way, you could produce 4 versions of your favorite fly, and carefully adjust your anchor according to stream conditions.