I've owned the cheap Tamron, the Kenko Pro and the Sigma EX 1.4x TCs. The Tamron was not quite as good, but many say that it affects the AF speed the least.
I took extensive real-world comparison shots between the Kenko Pro and the Sigma, using a Canon 70-200 f/4 L IS, Canon 300 f/4 L (non-IS) and the Sigma 100-300 f/4 EX. For each picture, I took the same shot twice, then swapped TCs and took 2 more with the same settings.
After all this I concluded that the only real difference between the 2 TCs was a small variation in colour of the images (as usual the Sigma gave a very slightly warmer image), and even that was only obvious when comparing side by side. I could genuinely not tell the difference between the 2 in any of the shots I took, except for the marginal colour difference.
Another thing I noticed was that there was no 'affinity' for the Sigma TC using the Sigma lens. The results on that lens were the same as for either of the Canon lenses.
I didn't notice any difference in AF performance, but none of my tests were difficult to focus on, so I can't reach any conclusions. I tried a similar test with the Kenko Pro and Sigma EX 2x TCs, however, and my lenses focussed much better with the Kenko. I didn't do any image quality comparisons, as the focussing performance meant that I didn't want the Sigma. I've heard (elsewhere) that the Canon TCs affect the AF performance more than the Kenkos, but I can't corroborate that.
And I'm sorry but no, I don't think I still have the test shots 
I've never tried the Canon I'm afraid - given the info I've seen, it didn't seem worth spending the extra.