Originally posted by biz-engineer a larger PDAF module that would be need for faster AF operation
That is a misconception. The physical size of the AF sensor unit does in
no way relate to AF speed. The latter is a question of processing speed.
The physical size of the AF sensor does limit the number and coverage and type of F2.8 sensors though, as these simply have to be far apart. So for a F2.8 cross type sensor with the added vertical line F2.8 spacing sensor you would have to make the AF module higher (y-axis) or use diagonal crosses (think Canon 7D). That will automatically require the lower camera body to be bigger/higher by probably 1-1.5cm as well.
For reference see the three F2.8 horizontal line sensors here:
https://us.ricoh-imaging.com/images/k1-landing-page/K1-AF-Sensor.jpg
Quite obviously turning them into vertical does not fit onto the current layout.
Remember F2.8 do
not in any way improve speed. They do improve precision as they capture farther apart "images".
Pentax themselves have in the past stated that they prioritized small body over larger AF module. It will be interesting to see if that is going to change.