HI Toshi,
The stuff re CA's and the Pentax 16-45mm was started by a review on Photozone.de. I find it quite humourous how the 16-45mm which was lauded as a great lens only a few weeks ago by many is now seen as a villain -a title it certainly does not deserve.
It is quite a thorough review which shows the sharpness that this lens is capable of and the excellent resolution it provides. Till this review though there was very little talk of a problem with CA and they actually state in the review
"To be fair the CAs are of the less hostile blue-yellow type (rather than red-cyan). In sharp contrast to the wide end CAs are almost negligible at the 45mm setting."
Link here
Pentax SMC-DA 16-45mm f/4 ED AL - Photozone Review / Quick Test Report
This is what Photozone has to say about the sigma lens you are considering
" This is clearly the weakest characteristic of the AF 18-50mm f/2.8EX - CAs (red/blue color shadows at harsh contrast transitions) are very high peaking in an average pixel width beyond 2 pixels @ 18mm. However, they're also quite strong occurrences throughout the rest of the range. CAs are quite easily correctable (e.g. via Raw Shooter Essentials or Photoshop ACR) but if you prefer straight JPEGs or don't want to bother with post-processing the issue will be quite noticeable in some scenes. Have a look at the enlarged sample portion (200%) to the RIGHT below.
It should also be mentioned that the lens exhibits also a fairly severe degree of purple fringing at large aperture settings which cannot be completely reduced during post processing. Unlike "normal" CAs purple fringing resembles diffuse halos which pollute into darker areas (see the image to the LEFT below). Stopping down helps to reduce fringing. "
Link here
Sigma AF 18-50mm f/2.8 EX DC Aspherical IF - Photozone Test Report / Review
Apparently the newer macro version is much improved in this area - but I don't know what the price difference would be.
All in all I think the 16-45mm is a very capable lens that achieves a lot for the price it is by no means perfect - there are some more expensive options that are better - comes down to your budget etc and what you photograph. If you do a lot of high contrast shots and use a lot of wide angle you will have some CA which you can post process very easily. If you are a pro for whom time is money - well in all honesty you should be spending the extra $$$ for a more pro level lens perhaps a Prime..