I have good friend who changed ist D to K10D this summer.
He has DA16-45/4, Sigma 70-200/2.8, FA43 and FA77.
Yes, AF of K10D is quicker and more "prehensile". On good level.
The main problem is that he can't shoot like with ist D - K10D's AF slide down to infinity very often.
It's annoying. Only auto selection works good. But he always uses center point.
Go to russian penta-club.ru - a lot of the same problems. Not 1 or 2.
I had BF with 43 and 77 at ist DS. All are adjusted with hidden engineer menu.
AF/BF/FF is head ache for a lot of users of Pentax.
I have friend with ist D who used screw-driver and made openings in the lower side of body to screw AF screws.
))