It's really hard to say which TC will be best with a particular lens. Ideally the TC would be designed to work with a specific lens and be optically matched to it, however that is rarely done these days (though Canon are coming out with a lens which has a built in matched TC as part of the optical design!).

I suppose that in theory the Sigma APO 1.4x should be the best match for the Sigma lens since if Sigma are going to optimize the design for anything, they'll optimize it for their own lenses.

I can tell you my experience with Tamron and Canon 1.4x TCs. The Tamron (non-SP) fives similar image quality with 300/4L and a 500/4.5L in the center of the frame. On a full frame camera the Tamron results is slight vignetting and corners that aren't as sharp. On a crop sensor camera they're pretty much equal.

In fact I use the Tamron 1.4x almost all the time when I need a TC on my 500/4.5L when shooting with a crop sensor body because AF is maintained and the image quality is almost identical to my Canon 1.4x.

The only Kenco I ever tested was an older Kenko 1.5x. It appeared to be absolutely identical to the Tamron 1.4x in terms of image quality and magnification. I suspect they were the same optics in a very slighly different housing. The new Kenco DGX 1.4x gets good reviews and it is a little cheaper than the Sigma.

I'm going to have to ask Tamron for a loan of their 1.4x and SP 1.4x TCs so I can see how they behave. The Tamron 1.4x that I have is probably 15 years old! However don't expect to see any test reults here soon since I have a couple of cameras waiting to be reviewed and I'm pretty much tied up with other stuff next month!

So here's an update on my converter solution. 

I order both Tamron converters from keh.com - they were used, but in excellent condition.  Very pleased with keh by the way. 

• The 1.4x is great and works without a problem on my 17-50mm f/2.8, 50mm f/1.4, and 70-300 f/4-5.6
• It did not work well with my Sigma 70-200 f/2.8 - optically it was fine, but the autofocus would hunt and never settle in. 
• The image quality when paired with my 28-105 f/2.8 was all around awful - almost irrespective of focal length
• I was surprised it worked with my 17-50mm but it did - it's a little soft at the short end but nothing too bad.  The 24-70 isn't a super useful range though so it was more for kicks.
• I am super pleased that it worked with my 50mm f/1.4 - it creates a very nice 70mm f/2
• I can stack the two converters for a 2.8x effect - the image quality does degrade as one would expect though
• I tried to decide whether to get the Kenko, Tamron SP, or Sigma to go with my Sigma 70-200 and finally decided to bite the bullet and get the Sigma.  It was well worth - the AF speed is incredible and the image quality very impressive.  I decided to only get a 1.4x converter though, when I want to use my Sigma as a 400 f/5.6 I'll just use manual focus. 

Thanks for your help!