I shot both days with the flash mounted. But was using the pentax 540, set to manual. So this different to your situation. I didn't face any problems with the AF speed or accuracy. Was just as fast and accurate.
On the second day, shooting in the foyer area, as the ambient light began to drop approaching early evening, the FA77 started to hunt and had trouble locking focus on a number of occassions. I then tried the DA*1650. Guess what? No hunting, no problem locking accurate focus :thumbsup: I will not be surprised that the K-5 AF is tuned more for the SDM than the old screw drive.
In CorneliusK's case, I think most lens will struggle to focus in the lighting conditions he was working under