matching memory speed grade and system board on ZBOOK 17 G2

[rajo]

I am thinking about adding more memory to my ZBOOK 17 G2 laptop, so I started to investigate and came across a strange information

Namely, according to CPU-Z, I have two modules of the type:

16KTF1G64HZ-1G9P1

These  modules are rated for 1866MHz.

Service manual of ZBOOK 17 G2 says that the laptop supports 1600MHz modules.

Are the present memory modules overrated? Or my laptop supports 1866MHz memory modules despite what it says in the manual?

Here is what CPU-Z says about Memory performance:

And here is an excerpt from the service manual

Lastly, here is info about my system board

The reason I ask is that I cannot easily source 1866MHz modules, but 1600MHz ones are readily available. If my laptop does not exploit 1866MHz modules to the max I would get 1600MHz ones. If the laptop actually maximally utilize 1866MHz modules then I should try harder to get two more of the kind.

Many thanks in advance

[Huffer]

The screenshot shows a 16 gb DDR3L-1866 memory module running at full speed I think. Your laptop has 4 slots and is supposed to max at 4 x 8 = 32 gigs of DDR3L-1600. It would not shock me if that premium machine can run memory at 1866 and yours appears to be doing just that but I am confused as to whether you have a 16 gb module in it?

[rajo]

I have two 8GB modules, and two more unpopulated slots.

If the system runs the present memory at 1866MHz then I guess introducing additional two sticks with 1600MHz grade will somewhat slow my system? I would go with identical sticks, but with different speed grade I could source where I am.

[Huffer · Accepted solution]

Yes absolutely if you put in 1600 memory it will all downclock to 1600 speed. 

https://www.ebay.co.uk/itm/CRUCIAL-8GB-PC3L-14900S-DDR3L-1866Mhz-204Pin-1-35V-SO-DIMM-Laptop-Memory-...