If it used exactly the same hardware, theoretically none. Even significant overlaps reduces the burden. Hence why TCL released the dtek60, the idol 4s and idol 4s pro on very similar hardware (not identitical). The issue is the drivers. If the drivers are all already written for win10m, its merely a matter of installing.
Specs are less of a concern. Its more about things like the gyros, internal disk, camera and camera optimisation, touch screen driver, sound driver, modem, that sort of thing.
If you've ever bumped around the drivers of a tablet, there's maybe half a dozen, to a dozen key drivers that vary across devices.