Some Intel rep posted:

Your processor is capable of supporting only 1, 2 & 4 GB DIMMs (to a maximum of 16GB); it cannot support 8GB DIMMs.
For optimal memory access performance, use three identical DIMMs in the (blue) Channel A DIMM0, Channel B and Channel C sockets.
If you add a DIMM in the (black) Channel A DIMM 1 socket, it must be identical to the DIMM in the (blue) Channel A DIMM 0 socket.
By 'identical', I mean same size, same voltage, same weight state configuration, etc. Optimally, you have purchased the four DIMMs as two matched-pairs (i.e. they are identical and have been tested together).
Adding a DIMM in the (black) Channel A DIMM 1 socket will indeed reduce memory access performance. In my opinion, the improvements in overall O/S performance as a result of having 16GB (instead of 12GB) of memory should easily offset the reduced memory access performance. I thus consider this upgrade to be a net win.
https://communities.intel.com/thread/116807