I've just transferred the data for multiple sites from the main disk of a web server to an NFS mount on a ReadyNAS NV+. Some sites exhibit problems while others work fine.
The permissions scheme is fairly simple. Each site has its own user and all the files are located together and owned by that user. Directories are set to 0750 and files to 0640. The Apache web server user (www-data) has all the site user groups as secondary groups, and is thereby given read access to all the data. Because "the world" has no access, sites cannot read each other's data, thus maintaining a reasonable degree of isolation. PHP scripts run as the site user, by virtue of Apache's suexec.
On the NFS mount, the ownerships and permissions were preserved. Not all the owners exist on the ReadyNAS, and some clash with users on the ReadyNAS. But I assume this is not important provided the client (there is only one client for this share) behaves consistently. The web server has the possibility of root access so as to be able to organise the files and directories.
Strangely, some web sites work fine, but others fail on file permissions. Converting the permissions to 0755 and 0644 seems to have cured the problem in all but one case, but I don't see why this change is necessary, given the group memberships of the Apache user and the fact that some sites are working happily with the reduced permissions. One site refuses to work even with the changes.