Just a few general facts about sub-d.  The Catmull-Clark sub-d subdivides the entire mesh, not just parts of it.  There are specialized sub-d types used by major CGI studio that can selectively subdivide a mesh but I don't see us getting them any time soon.  Sub-d adds polygons but it does not add detail, if anything it actually smooths out some details.  Take a cube for instance, add 1 level of sub-d and it clearly isn't a cube anymore, add enough levels of sub-d and begins to look more like a sphere.