Have you actually measured any performance difference?
According to my understanding, there should be no performance difference at all (hardware should not care). If there is, it would be caused by games & applications treating the iGPU wrongly, as if it was a dGPU and hence using that value in wrong ways that improve or worsen the performance.
~ 2,14ns difference from the CAS alone. Other timings are also shorter.
Given that total memory latency is often above 80ns not that much of a difference. And that assumes that AMD supports CL40 5600 Jedec. At least those Dimms should also support CL46 5600 Jedec, so that they should still work either way...
Intel supports routing all the available display pipelines through a single DP MST connection (any DP output). While AMD does not themselves publicly document this, they do support the same, just as Nvidia does.
Depending on involved peripherals, you might run into bandwidth limits. 4x 4K60 for example is possible over a single DP connection and Lenovo for example even lists that as supported with the right dock for their AMD laptops.
Is it confirmed that AMD supports 5600 CL40 anyway? Especially in notebooks without any overclocking features, you need to expect that everything is following AMDs spec exactly. Not like desktop mainboards that just on default do overclocks, even if they cause instability and will just try whatever settings the memory indicates as supported.
Secondly, GPUs are typically less sensitive to latency and much more sensitive to bandwidth (because they do more batched and pipelined processing where you have less surprising and truly random accesses), but a lot of them in parallel.
The CPU side of things is the one more sensitive to latency. For CPUs bandwidth, once there is a good amount, only becomes relevant in very specialized workloads.