Sorry to jump in on this: We are on 15.1 ( conservative in our upgrades) and have a similar issue.
We added the domain level signature, mapped it to users but no signatures showed.
Restarted the service and we got signatures on webmail but not via outlook
However i added a footer at domain level and get the footer in both outlook and via the web.No reboot required. The problem with footers is there are no domain variables available.
1 - Update the help to remind users that a restart of service is required to see the changed signature settings.
Any joy on the signature issues via non web mail (e.g. smtp / active synch?)