i'm not sure thickness of insulation by itself is a good measure of insulation power. not sure about anybody else, but somehow i feel warmer in half an inch of fleece than half of inch of down.
i think it's because not all dead space* is created equal. it'd be neat to run those sleeping bag tests on a down bag, a fleece bag and a synthetic bag, all of the same design, all of the same thickness.
anyway, it'd be neat to have another metric besides thickness to judge warmth. that could then be considered with respect to weight. don't get me wrong, i still think down would win, but i've a feeling not by as much.
*a space small enough that a convection cell doesn't develop.