From my use, there is no comparison between the new Pentax 645D and the Phase One DF body (or Mamiya AFD III I think it's still the III) in regards to
AF. The Pentax works like any other high end Canon or Nikon. It's fast and from what I tested very very accurate. The Phase One DF, is quite different
and may take some time to get to know.
With my DF the AF works in 3 steps, the first hit is never the most accurate, instead you get close, the 2nd press closer, the 3rd press is best it's going to get.
I also feel a modern 5.9K camera body, having only three points weighted to center is too few and out of date. There may have been some firmware updates
to the DF body on focus in the past 12 months. I don't have the latest firmware, but I don't own the vertical grip and thus can't apply firmware myself,
instead it requires sending the camera body to Phase One, not really up for that either.
No sugar coating, it's slow and tedious. If you are in the studio, it may work, for landscape, it's never been an issue as my subjects aren't going anywhere
in a hurry, for action I couldn't tolerate it. I have never tried AI servo mode and it very well might work better.
The good news, when it finally figures out where it thinks it should be, it's generally very accurate. Also the DF seems to handle low contrast subjects very
well, unlike my Canon AF.