Upon looking it up in the Dell service manual for this model, I found the spec sheet actually lists compatibility with both 3200MT/s and 2666MT/s. But of course the fine print had some caveats. Apparently only the SKU of this model that included an MX350 dedicated GPU has the privilege of running the memory at 3200MT/s.
Is there a technical reason for this or is it just another example of bullshit product segmentation? I can't imagine why the presence of a dGPU would allow for faster system RAM. That seemingly implies that the motherboard in my laptop is probably perfectly capable of doing the same, but has been artificially limited in the firmware. This really pisses me off because it's the iGPU model that would have greatly benefited from faster memory since iGPUs pull from the system RAM. They don't have fancy dedicated VRAM optimized for bandwidth.
It sucks that lower end laptops always have jack shit for tuning options in their BIOS. I feel like a lot of performance is left on the table when it comes to these things.
I looked into that a while ago because the CPU has a horrific habit of throttling despite only being at ~75c due to the strict power limit configuration Dell set up. There's nothing to do about either that or the RAM without going so far as modding the BIOS. They don't even let you undervolt anymore.