I tested the Pentax AF 1.7X rear converter with the DA* 50-135. I did two tests, one indoors on my bookcase, and the other through a double paned glass patio door on a small lawn ornament about 30cm / 1 ft tall. ISO was set to 1600, aperture to f/2.8. The aperture was reported as f/4.5 in the viewfinder and the EXIF data. The viewfinder also displayed the MF warning and the focus confirmation hexagon. The camera used the screw drive focus. There are no KAF2 contacts on the converter, indicating that the camera determines SDM by the presence of a signal at the power zoom contacts that are now used for SDM AF.
I prefocused by roughly turning the lens focus ring to an estimated distance. The AF 1.7X can only focus within a certain range; it cannot focus all the way from infinity to minimum focus. I set the lens to the infinity stop for the outdoor shot and to 3 meters for the indoor one. When I did not refocus within the range the 1.7X could cover, it ran once in each direction and stopped. I almost expected the camera to cycle through infinity to close focus (converter, not lens) over and over again but it just made the one pass in each direction.
The indoor shot has camera shake blur because I was too lazy to dig out the tripod, but focus was obtained with just two moves totalling about half a second. At ISO 1600, the exposure was reported as 1/2 sec @ f/4.5, which is pretty low light. I suspect part of the focusing problem was my shaky hands.
The outdoor shot was an instantaneous focus - well as fast as screw drive AF can go. There was no hesitation at all. I've included two full images and a crop from the outdoor shot.
All in all, a very usable combination. The Pentax AF 1.7X converter changes my DA* 50-135/2.8 into an 85-229.5/4.76 with more than acceptable image quality.