To change the visual appearance of an import (such as making spheres rounder for improved appearance or more jagged for improved performance) you can go the settings of the importer that you are using and edit the settings under Tessellation. An example of how these impact the import is that if you increase the Chord Height from 3.000 to 6.000 the object will have a reduced number of prims and therefore improve the performance of the scene but visually if the object was round it may appear more angled than before.
Applicable CAD formats
When importing CAD data it is usually one of two types - Parametric / Boundary Representation, or Tesselated.
These tessellation settings only apply when importing parametric data. If the data is already tessellated (the authoring application saved it as such), these settings will have no effect on the import process and the data will appear the same.
The following formats do not contain parametric data: 3DS, DXF, Blend, COLLADA, OBJ, Ogre, FBX, ASC
Other supported formats can vary and could contain either. Please refer to your authoring tools user manual or save / export settings to find out whether your data is already tessellated.