Extra ram always helps.
You have two choices.
1. Add a 2 x 8gb 3000 speed ram kit and abandon the 8gb.
2. Buy a 2 x 8gb 2666 speed ram kit with the same specs as your current 2666 ram.
It is likely but not guaranteed to work.
Assuming the two kits play nice, you will have 24gb.
If your current ram is a 2 x 4gb kit, all ram will operate in dual channel mode.
If you now have a single 8gb stick, 16gb will run in dual channel mode( flex mode)
and the remaining 8gb will be in single channel mode.
On balance more ram will trump faster ram unless you are using the integrated graphics.
Unlike ryzen, intel ram controllers are very good at anticipating ram use so ram speed is not all that important.
Read this study on ddr4 ram scaling.
Armed with a massive 32GB high-end Kingston HyperX Predator DDR4-3000 memory kit, we investigate how DDR4 frequencies and latencies affect real world performance on Intel’s latest mainstream desktop platform. October 13, 2015 by Lawrence Lee Product Kingston HyperX Predator DDR4-3000 32GB...
silentpcreview.com
spoiler... negligible difference in real app performance or fps.