We already set MAKO_MODULE_DIR to tempdir.mkdtemp_clean() in envs/common.py, so I don't see a case where this wouldn't already have been done.  Changing settings values makes me antsy, can we skip this logic?  I know it predates you, but I don't see a reason for it.  If we're nervous about removing logic that had been there, let's at least not modify settings.