Also, with encrypted volumes, just reading files on the raspi is very slow. Consider replacing raspi with odroid, utilite, or other fast, fanless, cheap arm sbc.

If the 'Pi is just a "cloud storage appliance", there's no need for it to care that the data is encrypted.

I use EncFS to encrypt on my laptops/desktops/servers/phones/tablets, and have BTSync sync the encrypted partitions to a machine at home that doesn't have EncFS installed. I'm using an old Mac Mini rather than a 'Pi, but the same principle works - the Mac Mini serves my archive/backup needs for my synced data (it's using software raid1 over two drives for reliability, and does TimeMachine backups to a rotating set of external drives for archiving), but none of the data on the Mini or it's drives is of any use if hacked/stolen, since if doesn't have EncFS or the crypto keys.