Let me help you with this information.
As you say, the processor you have supports up to 1333MHz. If Toshiba added a memory up to 1600MHz means that at the BIOS level there must be a configuration that will make the memory DIMM run at 1333MHz and not at 1600MHZ.
I would recommend you to contact Toshiba and confirm with them if by adding another DIMM running at 1600MHz the system will down clock the speed automatically to 1333MHZ or if you need to get a 1333MHZ memory. This is because Computer Manufacture set limitations in their units so I am afraid to say that we cannot over pass those configurations.
I think a 1333MHz memory should be OK but it is better to confirm with the Computer Manufacture before adding new hardware to the system.