Think pretty much what
@ANDARIAL said: unless you have a laptop with a totally unlocked BIOS (and the Dell 5515 isn't one of those), they only way to get better timings is to have a module which has this programmed on the actual module and
hope that the laptop even is able to support those.
Which is why I have sometimes seen people going on about certain SoDIMMs and being willing to pay extra for them - all because those SoDIMMs have specific timing programmed in. And AFAIR those timings have to be JEDEC not XMP.
As to why it would matter on a laptop: well if using the iGPU anything which increases memory performance is a big deal!