Anytime you mismatch memory, you run the risk of having issues, or maybe not even booting at all. You might add the memory and be fine, but you could also have some strange issues. There's no way to be sure until you try it, or you would just buy identical memory. MicroCenter's return policy is pretty good, though.
On a side note, are you sure you really need 16 GB of memory? You might not notice any difference, and could save yourself the money and the potential hassle.