I'll try copper in the matrix to see what happens; ceramic has also been suggested but I'm not quite sure what type of ceramic to use. The thermal grease factor also plays into the equation as well. I found that a thin film of vegetable oil in the mould does decrease the bubbles, but the film...
From what I've read, the thermal conductive properties of materials added to the matrix don't necessarily have a bearing on the overall silicone matrix heat conduction performance. Although I did consider copper.
I use RTV silicone rubber to make moulds for hard candy. Hard candy is basically boiled sugar that is poured into moulds at just under 300 degrees F. The castings of hard candy develop bubbles on the surface because the silicone does not dissipate the heat, unlike traditional metal moulds...