I'm pretty sure this is specifically why they specify what speeds of ram are supported. I don't see a reason why a slower ram card would slow down a faster neighbor. I can't say for sure if the board would prioritize the faster ram card but as long as you fall within supported limits of the board, you're gonna see a benefit.
On just about every laptop I've owned, they've come with two slots (2 GB) each and I normally only max out one of the two slots since 10-18 GB is plenty for me. That all being said, I've had no problems with this setup so far.
Mathematically speaking, there is benefit to having same sizes since you're not trying to manage a lot more memory on one slot while the neighboring slot (which is just as fast) only has to deal with a fraction of the ram usage.
All in all, unless you have a weird board that was designed oddly to sync the ram frequencies, there really shouldn't be a downfall to mixing two ram cards that work at different frequencies as long as that is technically the best that you personally are gonna provide the machine to work with.
Thank you, So at the current setup, it would get me no speed benefit for getting a higher frequency RAM (given that I have no experience in overclocking), but would it still be prudent to get the similar, but higher speed ram, for "future proofing", in case of later upgrades with more higher speed rams? – eBlaise – 2017-08-27T09:10:10.557