Originally posted by vandamro I'm not really sure why you would want to do that. The Sigma F2.8 70-200 HSM is not really well known for its image quality in comparison to tammy of the same spec. Moreover, the Sigma F4 100-300mm is known for fast focus and amazing sharpness. So that would be far better than the 70-200 plus TC.
You should try to figure out if you are going to the use the Sigma F2.8 70-200 primarily in its native focal lengths in the 2.8-4 aperature range a lot because otherwise I would just go with the Sigma F4 100-300mm.
I prefer the Sigma because it has HSM. I currently have a non-HSM version. I would like faster focusing. The Tamron 70-200 is still screw-drive (and has wicked fringing).
I don't want the 100-300 f4 because it can't do f2.8 (their 120-300/f2.8 would be great, but I don't think they do it for Pentax, and it's wayyyy expensive). Some games I go to under lights require f2.8, in which case I have to take off the TC and make do with only up to 200mm, and I still only get 1/350 ISO3200-4500.
As I already have the 70-200 and have used it, along with the TC, for thousands of images I already know what I need. Thanks for the suggestion though, I have often considered that option, as well as the Pentax 60-250, but again only f4.