I have a NetGear ReadyNAS Duo at home and I wouldn't dream of trying to multibox from it. It's just too slow compared with local disks.

When copying a large file to it, I get about 25MB/sec transfer over a gigabit LAN which is probably about one third of the typical speed of a SATA-150 hard disks. Access times seem pretty slow too. I expect that having multiple copies of WoW trying to read data from a NAS would be horrendously slow and laggy.