Hi Lukas/Pavel,
SSSD is now working correctly for us. Following changes were done:
1. We had to add "ldap_use_tokengroups = False" in sssd.conf file. This got rid of the SID's and now only the GID/group name is retrieved.
2. I believe the reason 'id' only pulled gid's for some groups without the names was probably because sssd was still keeping hold of the old cache. I had to manually drop the cache file (/var/lib/sss/db) and then restart sssd service. After this, the id command displays the data correctly. Had tried sss_cache -E but that didn't flush the cache it seems..
Thanks again for all the help.