Might have posted this before, but someone might find it of interest:
These informal tests were just to satisfy my own curiosity about lens sharpness with these TCs. I didn't concern myself with colour, noise, etc. All shot indoors with my K20D on my home-printed lens resolution chart.
These are all 1:1 centre crops. To be frank I don't see how anyone can compare TC results using 800px forum-sized images.
I moved the tripod back and forth to keep the fields of view roughly similar.
No PP apart from brightness adjustments so they all looked similarly bright.
(The smearing you can see in the right-hand-side converging lines near the numbers 14-16 and 18-20 is a result of the laser printer's limited resolution, not the lens)
Sigma 70-200 f2.8 Mk1, no TC
Sigma 70-200 f2.8 Mk1, Sigma 1.4x TC
Sigma 70-200 f2.8 Mk1, Sigma 2x TC
Personally I think the Sigma 1.4x TC is an excellent teleconverter. Bump up the contrast a bit and it's hard to tell it apart from pics taken with the lens on its own.
The 2x TC is not so great
Heres a version of the three above images shown at the equivalent of 800px forum sizes. I've corrected the contrast in the 1.4x and 2x versions here, but still no sharpening. (Bare lens on the left, 1.4x in the middle, 2x on the right):
And here's another version with sharpening: