I definitely agree with your observations that epoxy with cabosil is much harder to sand than either straight epoxy or the graphite infused epoxy. What is "best" in the end is a tradeoff of several properties, which is why I am interested in seeing what tests might have been done. For example, a harder shell of epoxy (e.g. with cabosil) might be very scratch resistant but could be prone to large scale fracture and catastrophic failure. My everyday example of this is the Corelle Livingware ceramic tableware- they are damned tough but when you do break one, it shatters into tiny shards. In metallurgy, it is common to increase hardness by controlled introduction of defects (for example by rapid cooling or by small % impurities) into the crystal structures, but the materials are generally more prone to shattering failure. So when we make epoxy "harder" by adding things to it, I want to try to understand the mechanism. In the bad old days concrete was strengthened by adding asbestos fibers (e.g. the Berlin Wall), and we do similar things with chopped glass fiber additives. Given the chemical bonding mechanisms that make epoxy hard, it is not a given that addition of particle impurities will necessarily improve overall hardness and strength characteristics.

I had read that the addition of graphite was to make the surface "slippery" (thinking of graphite like a dry lubricant) but I don't buy that model as the graphite particles are isolated and suspended in the epoxy matrix. Even now I am not convinced that graphite is doing anything other than colouring the epoxy black.

I guess I am a little surprised at the lack of hard data on this topic, as I would have thought that the epoxy manufacturers would be all over this as a way of getting different characteristics, as a way to sell the methods to the different users of industrial epoxies.