I pulled Arch's libinput PKGBUILD and changed the version to 1.5.901 and rebuilt, installed, and rebooted.
The various accel profiles exposed in Gnome clearly have a more pronounced effect, but the mouse speed is much, much slower than with 1.5. Annoyingly so, even with it turned all the way up in Gnome settings.
this seems to be a hidpi screen issue, same as bug 99383
*** This bug has been marked as a duplicate of bug 99383 ***