Tried plaster cores for homemade piston production an found two shortcomings. One, even when baked in the domestic oven, the plaster never shed enough moisture. First attempts had the cores venting enough residual steam the sprue erupted like Mount Vesuvius! Further baking still tended to have the cores crack when the aluminum was poured. Second, the plaster was not very permeable so there was nowhere for the gases near the surface of the casting, or that generated from the plaster, to go but into the solidifying aluminum making the surface porous. Mind you, these were larger piston cores for a vintage engine that were about four inches thick. At model engine scale the plaster will be a thinner section and easier to dry clear through.
I do not know if the composition of the investment slurry differs enough from plaster that it is more permeable to foundry gases, but certainly the high burn out temperature seems to solve the residual moisture problem.
-Doug