I figure this will probably be shit on here, but have you ever played with Windows Server Deduplication? We use it all over our corporate Windows environment, with a distributed file system, and it averages 48% globally. It doesn't chew up RAM, integrates with shadow copy services, behaves itself while it runs in the background, and I honestly don't see much if any performance hit reading/writing. Now we're not talking petabytes of data, but each of probably a hundred file servers worldwide has 10-20 terabytes worth stored.

It's literally set it, and forget it. While you gain 50% of your storage back.