> Should adding tests for the report methods be a separate issue from this enhancement?

Yes, I would create separate issues for this.  I would even recommend separating this into one issue for the report() methods (could be done first), and another issue for adding tests for additional directory levels (see additional note below).

> Any deeper testing of dircmp will also have to not be necessarily comprehensive, as the permutations of test inputs start to become too time-consuming. setUp and tearDown may also need some refactoring for deeper testing.

For testing additional directory levels, you may want to wait until after issue 15403 is complete, which is slated for (hopefully) shortly after the 3.3 release.  That issue will add functions for the easy creation of arbitrarily nested directories of files, etc.  You will see I made a comment there about application to the filecmp tests.