I have found from a lot of experience that except in deep shade (perhaps a forest where the trees block out a lot of the light) the AF performance is not an issue with the 2X TC on my Sigma APO 70-200 F2.8 EX (origonal - non macro non DG rear element coated).
It works just fine! as does the 1.4X TC. I have both sigma TCs.
A couple of things to look out for are specifically with respect to metering.
I have found on my K10D that the addition of a TC modifies the metering and the K10D tends to over expose with a TC attached. WIth the 2X TC I use -1.3 stop (i.e. under expose) on the compensation to get back to correct metering.
The metering issue is a long story on the K10D, related to maximum apature and focusing screen. WHat ever you do, test your camera against a uniform target with and with out the TC on the lens and make sure you know how it behaves before you go out shooting.
As for image quality, see below.
https://www.pentaxforums.com/forums/187646-post19.html https://www.pentaxforums.com/forums/208249-post1.html