Modern RAM controllers allow mixtures of RAM to work under a variety of conditions. The downfall to this is RAM has a maximum speed it can operate when everything is identical. When everything is not identical RAM runs slower. If you want the best performance at the cheapest cost, figure out which slot the 8 GB module you have now is in, B or D (you can do that with the above command), and then install 1 of these modules, under the back cover in A (if other in B) or C (if in D).
I am looking to understand if configurations not mentioned in the manual are allowed. For instance, can I have 2x4GB in the primary slots (under the keyboard) and then add 2x16GB in the secondary slots (which are more easily accessible from the bottom). This would be 40GB total which is not explicitly stated in the documentation.If I were to do this, I would make sure the timing spec matches.such as 2666.    
Precision 7740 memory configurations