Unexpectedly, ownership of Total War: Warhammer has started showing up on public Steam profiles before release. So with the caveat that the margin of error on these figures is likely to be high, here's how preorders look to be shaping up at present, compared against post-release sales of other Total War titles:
You might also be interested in alQamar's thread: TW Warhammer reviewers score hunt - comparing Rome 2 and Warhammer reviewer scoring over time.