Hi Nick, I'm following your build with interest. Looking good so far. I like the manual scraped cast iron parts :)

Reading the Silimix-282 description I found that the company did exactly what the forum guys were trying to achieve. To have a very compact aggregate to use as little resin as possible. Less resin means higher stiffness. But if you have an internal metal structure or use the resin to dampen vibration in a cast iron machine then stiffness is not very important and more resin means better vibration damping.
It is good for DIY that such aggregate mix is available on the market but what if you want real granite aggregate instead of quartz? :) You have to go back to that messy thread trying to find answers/recipes...

I'm not sure if/how I'll be using epoxy-granite or cast iron in this build. I cannot add to much weight. For cast iron it should have at least 10 mm thickness for a decent stiffness and for epoxy-granite about 25 mm which translates to roughly the same weight.

Like m_c said, I need to find the best compromise. I'm aware that I cannot have great material removal rates, surface finish and tool life with a lightweight machine.