I'm just resurrecting this old thread to express thanks to Sigma and Maxwell 1295. I had a wonderful experience with walk-in service at their repair facility in Ronkonkoma, LI, near my home. I had a a 24-60 2.8 that seemed to work pretty well on my old K100, but just didn't hack it on the new K-x, which seems to be more revealing of flaws in IQ. Also a bit of of front focus issue.
Anyway, I read about the facility here, and drove out for a look (15 minutes away, eat your heart out). The tech took a look at lens on the bench, said it was front focusing, and basically unrepairable--it couldn't be rechipped since the 24-60 has been discontinued. I was offered full credit towards purchase of a new 24-70, but when it turned out they didn't have any new in stock (they don't sell lenses here) he offered me a "nearly new" unit in perfect condition as a even swap.
Then came the really cool part of the deal: he said if I left the camera overnight, he'd calibrate the lens to my body. Sigma, as you know, has to reverse engineer the Pentax lenses, and little niggling incompatibilities can arise with new models. The stuff you buy is calibrated to a generic K mount; you can send in a lens and specify you camera model, at which point their use that specific model for calibration. Or you can ship them the lens and the camera, for an absolute perfect match. You can read a more technical explanation of all this at
https://www.lensrentals.com/news/2008.12.22/this-lens-is-soft-and-other-myths
Anyway, what I got back the next morning was like a new camera. Nice sharp focus, as I expected. What I didn't expect was that the focus speed improved dramatically. Absolutely snaps and locks on instantly. Particularly interesting--if you look at the reviews on Fred Miranda, the 24-60 is generally ranked a little higher than the 24-70, mostly because it is said to be slow to focus. Well, not my copy.
Definitely read "This lens is soft and other myths."