I don't know silicone's thermal transfer properties, but sure are horrible.
But very few pople remember that mosfets already have a plastic package, with a bad heat conduction, so theres no point in attaching some high performance material on top of it. The ideal surface for cooling them is in contact with the mobo, wich is also made with plastic and some thin metal that improve thing a bit.
That means the area available for cooling is mostly plastic, so using silicone should not be so terrible. My idea is that the huge contact surface of this "block" will compensate for the low conductivity of the material.
Stay tuned...
