In this generation of MB and CPU the memory speed would drop based on the number of chips installed due to voltages provided to the memory. I do remember seeing that fully-loaded memory configurations would drop to 800mhz from all major vendors. I am trying to find references in either the Dell manual or others for this, but it does take a lot of power to run all of the resources. I will post something if I can find it.
One thing to note, the manual says that you can't run these things in a full configuration with all 4 nodes and the 1100W power supplies (full disk, memory, CPU). Not really sure what that means to those wanting to run full disk stacks. In the HPC model you would run them disk less or one set so never fully loading them.