Sell the 8GB and go with 3000mhz 16GB of RAM (preferably CAS 14 or CAS 16).
It is not true that typically doubling the amount of RAM you have is better than slightly quicker RAM. It all depends on how much you need for your programs.
So say you have 32GB of RAM and upgrade to 64GB of slower RAM, the 32GB RAM system is going to be faster in gaming if all you do is play video games. As an example.
Not to mention that there is a huge chance mixing the 8GB kit with the 16GB kit will not even make the computer boot up.