The Dell certified maximum amount of RAM you can have is 8 GB and you are already at the maximum.
If it is working with 1600 RAM then I assume it is actually running at 1333 as the system would clock it down. So again, the Dell specs are 2 SODIMM memory slots. Each slot can hold a maximum of a 4 GB RAM module for a total of 8 GB in the system. There have been users that claim to have installed 2 x 8 GB RAM modules for a total of 16 GB. But there is no guarantee the system would boot or be stable.