No, the memory is unified and BOTH, the CPU and the GPU are able to access the full 16GB of the memory, no matter if fast or slow.
The thing is, that the GPU needs higher bandwith than the CPU (on PC the CPU ram is way slower than on consoles), therefore as a dev you will try to keep the GPU specific stuff in the fast area and the mostly CPU stuff in the slower part. Stuff which both access (there are reasons why you want to have unified memory) you try to keep in the fast area as well.
The RAM is using the same lanes in the computer, so you access either the slow part or the fast part of the memory, not both at once. You can read this as accesses to the slow memory slows down access to memory overall...