no, don't sub powdered for superfine!!! Powdered sugar (icing sugar) also contains a bit of corn starch to prevent lumping!
RL Berenbaum (the Cake Bible) wrote this article
about sugar... which should tell you everything you EVER wanted to know...
if you can't find superfine, you can run the extra-fine through the food processor to get it close to the right comnsistency!