Yes, as far as I can tell all of the owner and group information is preserved is the -a option. When I "ls -la" the alf_data directory, for example, everything belongs to the root user and group except the postgresql directory that is owned by postgres but in the root group.
You should take care with the way you restored the extension folder, as there are files in there, like share-custom-config.xml and alfresco-global.properties that have adresses/hostnames/IPs in them, if you blindly copy them, you can make your restored server to connect into the original one.
For thoses cases, you better to recover the content, avoiding this kind of problem.
To mitigate problems with these properties dependant of your environment, you may include them in your startup scripts as part of JAVA_OPTS (ex. -Dalfresco.host=xxx). And so the essential config under shared (at least for alfresco-global.properties) is the same for every environment and so, in a backup env too.