NAS
NAS doesn't have a Windows OS
NAS will always be considered at Network Drive,with SEP you can enable SCAN network drive, it will scan the mapped portion of the NAS but will not take any action on the file on NAS, it will prevent the file copying to the client machine.
For NAS you SPE (a.k.a Symantec Antivirus for NAS) which a different product all together, you cannot run a scheduled scan on disk with terabytes of data, also SAV for NAS does multi threading i.e. it will scan many files being accessed simultaneuosly without impacting the performance of NAS
SAN
SAN is considered as Local Fixed Drive and so there is nothing different between a SAN and HDD.
For reducing DISK I/O on SAN you can look for Virtualization best practice using SEP 12.1.
http://www.symantec.com/business/support/index?page=content&id=TECH173650