I've owned both the 16-50 and 50-135 since soon after their release. The 50-135 is excellent optically and mainly would benefit from a newer auto focus motor. However, if you really want to speed up its focus speed, you would have to shorten the focus throw. I just looked at it and the focus throw is about half a turn around the barrel -- the only lenses I have that have longer focus throws are macro lenses and, oddly, the DA *55. The 16-50 is pretty fast to focus, but really could use some work optically. It has a fair amount of distortion at 16mm, has a lot of purple fringing, and edges are soft wide open.
At the same time, there is no real information right now. Asahiman did say something about Pentax working on re-designs of the DA * zooms and it would make sense based on their age, but since then, most of the information leaking out seems to be about full frame lenses and filling out the DFA line up. My guess is that they are working a bit on this project, but as with many things, it is under-resourced and will take awhile to come to fruition.
Pentax is pretty tight lipped about future developments -- other than the chart they put out with regard to lens development. Probably the first sign that they are releasing new DA * lenses will be sales from B and H and similar retailers to clear out old stock. That's the way it was the DA limiteds when the HD versions came out.
---------- Post added 12-07-18 at 06:03 AM ----------
Originally posted by Lev Very easy! First thing they should lower prices a little bit more for current lenses that requires redesign so it will be more attractive for people, this way Pentax will get some amount in short period of time, well some amount. Start developing of new versions with same glass but new motor and electronics. We will get something out of this world! Imagine 50-135 with fast and reliable auto focus, BOMB! This will be just unbeatable piece of equipment.
I think at current prices (700 for the 16-50 and 800 for 50-135), they are probably as cheap as they will be until sale prices clearing stock. If Pentax releases new versions, I would expect the new versions to be a bit more expensive, not less.