The problem I see with the 841 is timing.  If it had come out a year ago, or it had done 18+ TH, it would be awesome.  But it's taken 1.5 years since the S9 for canaan to come out with something that just barely beats it.  

Presumably Bitmain has also been working on something quite a bit better than the S9.  And we all know how they like to crush competitors.