Assuming that an added 16gb is possible, that is likely good.
You will have 24gb to work with.
The matching 8gb of the installed ram will operate in faster dual channel mode of the first 8gb of the 16gb stick.
That is called flex mode.
Performance will be better, like perhaps 10%. If you are using integrated graphics, the graphics will run considerably better.
You may also be able to dedicate more system ram to graphics vram use.
All ram must be run with the same specs of voltage timings and speed.
Faster ram will slow to the speed of the slowest.
I suggest you go to a ram site like kingston or crucial and access their ram upgrade app.
Enter the make/model of your laptop and you will get a list of supported upgrades.