I'd like to see the Sigma 70-200/F2.8 tested on a Pentax. I have both Mk II non-OS version of this lens and a Pentax DA* 60-250/F4 and I continually switch between the two.
The 60-250 has weatherproofing, better range at both ends, less CA, and is lighter, but it extends so it's physically more bulky.
The 70-200 has an extra stop, does not extend so it's more compact, has a bit more CA, and I think it may be sharper. It also has a unpleasant character to the OOF background in some shooting situations. Look at the quality of the bokeh in this shot:https://dl.dropboxusercontent.com/u/51093924/K5_15351.jpg
I shoot junior sports with both, switching between them based on subject(s) composition (do I need shallow DOF or not), light level, shooting distance range for a particular event and the weather.
I still have not been able to come to a decision about which one I prefer. I think a comparison between these two and the cheaper Pentax 55-300/f4-5.8 would be informative.