Shared Insight Cache
Taken directly from the 'New features' blurb: Built for Virtual Environments: Integrates with VMware vShield Endpoint to offload critical scanning while providing the strongest protection for your virtual infrastructure. Symantec Endpoint Protection can white list baseline images, maintain a shared scan cache, randomize scans and updates, scan offline images and automatically identify and manage virtual clients.
& this: Utilizing a SIC server can reduce the impact of full scans by up to 80%, but does not significantly reduce the impact of Active scans.
& this (the above 80% refers to this paragraph): The Shared Insight Cache tool improves scan performance in virtualized environments by not scanning files that a Symantec Endpoint Protection client has determined are clean. When the client scans a file for threats and determines it is clean, the client submits information about the file to Shared Insight Cache. When any another client subsequently attempts to scan the same file, that client can query Shared Insight Cache to determine if the file is clean. If the file is clean, the client does not scan that particular file
Another tool to use is Virtual Image Exception Tool.
What's new in SEP 12 has a section specific for virtual environments.
How Insight Lookup works explains some ideas on not having to scan files over & over again.
Virtualisation best practices speaks for itself.
About Shared Insight Cache.
I know none of these specifically address your large volume question, but should go a long way in addressing your overall concerns.