I picked the camera up today and I must say, I'm furious. The comment from them says they upgraded the firmware. That's it. Did it help? Nope!
See for your self's:
Center crops:
2014.07.03a - zafariqbal
Uncropped:
2014.07.03b - zafariqbal
The galleries include all but a two shots taken since I got the camera back. The two excluded shots were accidentally shot @ f6.3 (though one of them still looks slightly out of focus).
Center point used - no recompositioning or anything like that. I was careful and extra slow when focusing and taking the shots, and I re-re-focused plenty of times.
I'll mention again that the lens used (Sigma 17-50mm f2.8) is fine. In fact I used it a lot recently with my other K-30 body and shot during an entire evening and under different lighting (varying natural, few different kinds of artificial and candle), and not once did my mind wander towards the idea, that some shots were off focus because of the camera or lens.
I tried to make it clear to them, that the problem can be illusive, hoping it would make them diagnose it better. I can't express enough how disappointed I am. Now I have to send it back in again, be without my backup body for another couple of weeks.. and MAYBE they will fix it this time.