Actually, I do that exact thing. I run a weekly backup of my DNS-323 to my DNS-321 using a program called Beyond Compare. It does a synchronization of about 800gigs of data, any changes are written to the DNS-321, and any files not present on the DNS-323 are removed. The average throughput is over 10mbytes/sec, and some of that is attributed to a lot of small files, which are much slower than large files.
The network is configured for 4K jumbo frames and of course gigabit connections.