Well, your board's manual clearly shows the "Load XMP Setting" in the OC Tweaker menu, in the DRAM Timing section:
Select the 1600 speed profile with that option. Loading the XMP profile is the standard way of setting memory to its highest specified speed.
You could also use the DRAM Frequency option to choose 1600 if you want to do things manually.
ALL memory will run at a default, low frequency when first used in a board. The memory manufacture has no idea what board their products will be used in, or if the PC is capable of running the memory at a higher speed. The default lower speed is used to insure the PC will start when the memory is added to the PC, rather than failing at a high speed. This saves everyone from grief, when the user is not exactly sure what to do.