I think high-speed ddr4 might still be somewhat problematic. About 99.9% of all ddr4 modules over ddr4-2666 (or rather ddr4-2800) are factory overvolted (1.35V instead of 1.2V) (even if they often list 1.2V, which they'll do on standard frequency, not their advertized speed). On the desktop, nobody ever cares, but for notebooks I'd suspect this is quite a problem.
That said, ddr4-3200 modules at 1.2V seem to exist now (they are all quite recent additions as far as I can tell). Kingston for instance has hyperx fury (dimm) and hyperx impact (so-dimm) which are explicitly saying they can do ddr4-3200 at 1.2V, if their website is to be believed... But even if it can do it, such expensive memory might really not be what a OEM might want to include...
So I suspect the fastest you can realistically expect is ddr4-2666 (hopefully with dual channel or at least an empty memory slot...).