Does the density of the brushes used in a DC motor increase efficiency?
Several aspects of brush design have some effect on DC motor efficiency. That would include friction, brush resistance, and contact resistance or contact voltage drop. Those items would be partly determined by brush dimensions and material. The effect would be influenced by armature voltage, current and operating speed and perhaps some other factors.
For carbon brushes, brush resistance is simply the resistance of the brush material. The resistivity is determined by the composition of the brushes, graphite, a binding additive and sometimes copper powder and other metal powders. Some very small motors have spring brushes made of metal alloys.