This is what I have been saying, all different versions of the Pentax 18-55 are not optically identical, at least the outcomes are not, in contrast to what Bojidar tells in his K-mount page. In fact, I could see that the QC standards of most of those plastic (DAL) lenses are just inferior - as I have owned 3 different copies of it: the best one is that come with my Olive K-m body, which performs the best and had no workmanship issue of any when it was born from the factory, unlike the other two.
Here is the proof again, with photos shot at the same time on a test chart. The remarks made by the OP are clear and self-explanatory:-
For quick reference, below are the directly linked pictures from the DAL and the DA II:-
(Above: K-x + DAL 18-55, Click to Enlarge.)
(Above: K-x + DA 18-55 II, Click to Enlarge.)
And, quoted, for what the OP observed (and it's so obvious indeed):-
"First, the DA 18-55 II lens. Note the better edge and corner sharpness compared to the kit lens supplied with the K-x, the DA L 18-55...
The image below is from the L lens, and is not corrected. The two correction functions don't help to correct edge focus - well, if they do, it's not noticeable in my images. I tried all variations.
Even the centre focus drops off towards the edges. Oh, and it's even worse wide open. I'm really trying hard to be nice to Pentax here... "
In short, those newer "plastic wonders" of Pentax could suck! And that all depends on your luck, no wonder! :-( ;-> And afterall, the Pana delivers the best result!
"The image above was auto-corrected in cam; the two settings take around 5 seconds to process and correct for curvature and CA. This slows the camera down immensely, but produces a square, almost error-free image."
"Panasonic get criticised by many for including geometry and CA corrections in-camera - they are applied in every shot. The wonder of all this is that it takes no time at all, in a Panasonic (not 5 seconds like a Pentax), to process these corrections."
As I also have the Pana GF1, I can firmly tell that what that guy says is true!
Some brand defenders would yet argue that the K-x is not the uppermost model of current Pentax DSLRs, so they should see what a new K-5 user reports on the same issue, below!
K-5 Continuous Shooting Does 3 Frames Only! If and When..
Standard Zoom Mega Shootout! (DA Vs FAJ Vs DAL Vs SA Vs FA!)
(AF) Lens Matching with the Navy K-x