http://www.pentaxforums.com/forums/pentax-k-5-forum/136324-k-5-1-03-versus-k20d-auto-focus-tests.html
The results are self-explanatory. Testing conditions including lens in used and the lighting level is stated clearly below each pair of the comparison test pictures.
It can be seen that the old K20D K.O. the K-5 with the latest "improved low light AF accuracy" firmware 1.03 hands down! :-o
It seems that Pentax has gone back to the old *ist D days for poor AF accuracy with their latest SAFOX IX/+ used in the K-r and K-5, although in speed the new 9th generation of the Pentax system seems to be faster! >:-(
Nevertheless, a few other K-5 users have carried out some random ad-hoc checks on the AF accuracy of their K-5 units at lower light. It seems that some improvements over the old firmwares are shown but I can still see some OOF samples:-
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=37937710
So, the quick conclusion is that the new K-5 firmware could improve something but if it is compared to some old Pentax DSLRs like the K20D, the AF accuracy of the K-5 still, sucks.
My personal experience is that my K-m has the best AF accuracy, which has a mature SAFOX VIII. In fact, I found that the AF accuracy of the K-x had deteriorated but just have never imagined that the K-r could be even worse! Too bad!
Related:-
"Hysteria"!
Improved AF Accuracy of the K20D
AF Speed Difference of K-m Vs K20D
K-x' Time Longevity on AF Accuracy
K-r's AF Accuracy Affected Much by Colour Temparture of the Light Source
Magnified AF Errors of the SAFOX IX than the SAFOX VIII