While this may not be a 'typical' result from using the 70-200 + TC combo, it is the reason I caution against that selection if you intend to shoot "long" a lot:IMAGE LINK: http://www.flickr.com/photos/snydremark/5329626868/ 70_200MkII_2.0x-0455
70-200 f/2.8 MkII + 2x TC MkIII (tried 2 different copies of the TC with the same result)
, on Flickr
100-400:IMAGE LINK: http://www.flickr.com/photos/snydremark/5329015605/ 100-400
, on Flickr
Both were shot from a tripod and manually focused in Live View at 10x magnification, to get each one as crisp as I could.
Any time you are throwing an extra TC in there, you have glass that wasn't expected in the light path. Given the gulf of variation that occurs in the hardware, there isn't any way to know whether you're going to get good results or ones like mine. If you get results like I did, then troubleshooting gets trickier; because now, instead of two variations to go through, you have six.
The combo is nice if you're only shooting beyond 200 once in a while; but if you plan to be beyond that very often, one of the natively longer options is a much better bet. Alan threw out some good alternatives to contemplate in his earlier post.