All Canon extenders mount on the Sigma 70-200 2.8 OS. I used to have 4 different 2x TCs, including the expensive Canon 2x III. Sold it eventually when I got my Sigma 2x APO EX DG. Sharpness in the center was close, actually favored the Sigma a bit. Corners: Canon won by a bit. What the Canon does better is controlling CA. I ended up selling the Canon and freeing up some funds this way.IMAGE LINK: http://www.flickr.com …/29108710@N04/6982833742/ Great Egret with fish 6
However, the question is what are you planning to shoot? If mostly birds, I'd go longer, such as Sigma's 150-500 OS, or perhaps the Canon 100-400L IS. Yes, it is only 400mm but a good copy is nice and sharp and contrasty. The Canon 400m f/5.6L is good too, although no IS.
If you think that you could use a fast 70-200 lens in many applications, it is worth considering the Sigma 70-200 2.8 OS or (if budget allows) the Canon 70-200 2.8 IS II.
I'd say a Sigma 2x TC works pretty much equally well on the Sigma lens, but if you go the Canon route, I'd get the Canon 2x III.
The Sigma 70-200 2.8 OS is decent with the 2x Sigma TC, I usually avoid wide open (f/5.6) it is much nicer at 6.3 and up. Still, even an average copy of a 100-400L would beat it in IQ and especially AF speed.
I also own the Sigma 120-300 2.8 OS which is a great lens. Sharp, and fast, and works great with TCs. But it's quite heavy. So quite often I actually take the Sigma 70-200 OS + a couple TCs with my for a "nature oriented" walkaround. When I know that I'll be sitting in a blind or other stationery places, I take my 120-300 OS and tripod/monopod.
but again the bottom line is, do you plan on only shooting birds, or do you also shoot events, portraits, sports, especially in lower light environment? If yes, then a good 70-200 2.8 OS/IS + TC would be a good, flexible solution.
Here are a couple shots taken with my Sigma 70-200 OS + Sigma 2x TC (cropped somewhat and processed):
(Note: the last one was taken using an older, non-reporting TC, hence the EXIF info stating 200mm)
, on FlickrIMAGE LINK: http://www.flickr.com …/29108710@N04/6941296912/ Great Horned Owl Babies 13
, on FlickrIMAGE LINK: http://www.flickr.com …/29108710@N04/6193055305/ Egret with Sigma 70-200 OS and 2x TC
, on Flickr