There's some relevant information in this thread.
K-1 Automatically stops down my Pentax-a 50mm 1.4 in live view. - PentaxForums.com
My understanding is that Pentax AF is calibrated fro ƒ/2.8.
The issue with a wide opening lens is that it's anticipating ƒ2.8 DoF when locking focus, so the camera may choose a point somewhere within the 2.8 depth of field that is not exactly where you would have put it even though to the AF system it will be within acceptable focus.
But practically, shooting with mh DA* 55-1.4, the theoretical problem has never translate into an empirical issue.
You focus on the curly wire fence junction, you get the curly wire fence junction in focus in my experience, even at ƒ/1.4
If we listened to everyone who tells us what we can't do with our Pentax gear, we'd be missing out on an awful lot.