It is a power restriction only.  If you take in account for memory, additional PCIe cards, and 16 drives, each requiring power, is the only reason for the restriction on the configuration.   It may work at the moment, but for how long?   
Hi, Is it possible to place 16x2.5" drives with 4xE5-4650 CPU. When configuring the R820 with 4xE5-4650 CPU it says, the maximum number of drives is up to 8.
R820 Using 16 drives with E5-4650 CPU