Originally posted by AlbertSiegel ...does it kill the lens AF in favor of the adapter AF?
The lens's AF is totally disconnected from the camera. In other words, the AFA takes over the AF.
The AFA performs AF by moving its optic elements. Thus its AF has a limited range. For example, if you leave the lens at infinity, the AFA can't focus at something at minimal focusing distance.
So you have to "rough" focus by manually focusing the lens, then half press the shutter release for the AFA's autofocus to take over. This sounds like a big hassle. But in practice, it becomes second nature and you can do it without thinking.
For the FA 300 you mention, you can use a Tamron 1.4X or even 2X TC. If you can find them, they are sure less costly than the Pentax 1.7X AFA.