I can't answer for this exact combo but the following may help:
I sometimes use the Sigma 2x on their 70-200mm f/2.8 EX lens, and the results are generally OK but can be a bit patchy. The 70-200 is a jolly good lens, almost but not quite as good as Canon's 70-200 2.8L, which is the gold standard for tele-zooms. Putting their 2x extender on it results in a 140-400mm f/5.6 lens which is still very useable, but with some limitations. There is some loss of sharpness, but the combo is still adequate for most purposes. More serious (for me at least) is that the bokeh is not as good, and there is a noticeable loss of contrast in challenging lighting, e.g strong backlighting.
A couple of examples to show what I mean, both taken with the above combo:
[attachment=18038:IMG_2102.jpg]
The above is an example of the combo performing quite well. You can't really tell from a small JPEG, but I can assure you this one is razor sharp, and the in-focus Goliath Heron just pops out from the similarly coloured and lit background on a 13x19" print. But you can also see that the bokeh leaves something to be desired.
[attachment=18039:IMG_2893.jpg]
This next image isn't so good, and I wish I had shot it with my prime 300mm f/4. Although the fur of this handsome young fellow is in critical focus and there is no camera shake (you'll just have to take my word on this) is still looks a tad mushy even after lost of post in Lightroom. This, I think, is due to the loss of contrast in a horribly backlit situation. Also, note that the bokeh is downright unpleasant.
[attachment=18040:IMG_2159.jpg]
But then again it doesn't always behave that way in backlighting, and sometimes everything is just fine... Odd, but I'm not complaining.
As I say, this is not the exact combo you are asking about, but I hope this is of some help.
Ed