Short answer... No benefit.
At the same speed, lower cas is better.
But, that does not translate into a meaningful fps difference if you are using a discrete graphics card.
Here is a study:
http://www.silentpcreview.com/article1478-page1.html
Perhaps of more importance is your ram upgrade plan.
If you are thinking of simply adding ram, reconsider.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
If you buy higher speed ram, it will operate at the speed of the least capable kit.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.
If you do buy more disparate sticks, they should be the same speed, voltage and cas numbers.
Even then your chances of working are less than 100%
What is your plan "B" if the new stick/s do not work?
If you want 16gb, my suggestion is to buy a 2 x 8gb kit that matches your current specs.
Then, try adding in your old 8gb,
If it works, good; you now have extra ram.
If not, sell the old ram or keep it as a spare.