I don't think you will get any performance difference.
But... adding 2 disparate sticks may not work. Ram must be matched.
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.