So, what's everyone's opinion on this?
It's always been my understanding that confectioner's sugar (also called icing sugar or powdered sugar) contains a small percentage of corn starch.
Regarding it's use in wine, I would be wary of using volumetric measurement since it's more dense than granulated sugar. I tend to use measurements as a guideline anyway, and I add sugar until I get the desired SG.
One way to come up with a conversion would be to weigh a cup of each type of sugar and figure out the amount that way.