Well, for starters, you are giving up half of the memory bandwidth. Purchase another DIMM identical to the one that you have and put it in the primary socket for the second memory channel (you likely have two Blue and two Black DIMM sockets; you want the two DIMMs in the Blue sockets). Alternatively, replace the one 8GB DIMM you have with two 4GB DIMMs and put them in the Blue sockets.
Now, as to memory speed, provided that your motherboard supports it, you can run memory faster than 1600MHz. Understand that, technically, anything over 1600MHz is overclocking your processor, but this is minor and likely will affect temperatures by only a small amount. Again, your motherboard must support it.
Hope this helps,