For the XIM its meanigless since it polls at 1000hz (independent from the controller polling rate so it wont matter if next gen controllers still run on 800hz or less). Also i doubt you can tell a difference between 8000hz and 1000hz given that you cannot tell any between 1000hz and 500hz.
As the person stated in the video it can be helpful to reduce button activation times but you dont need a 8000hz mouse for that but proper switches/activation registration plus maybe a software solution to avoid double clicks.
Until reviews are out and this turns out to be noticeable, which i very much doubt, its just a PR stunt to have another "big feature" they can advertise.