I can't totally answer your question, but i recently got tamron's regular 1.4 and 2.0 and they will let a canon rebel AF, even when stacked. Canon's converter will not, so i hear, and it will not work on all lenses, whereas the tamron will. I have not tested focus speed, but canon claims it will slow with a TC in use (maybe only theirs?) Everyone says the image quality drops with 2.0x converters..i cannot see any issues with my tests, but they were not comprehensive.
I was planning on getting 'better' converters but a friend was selling these (to get the canon 1.4x) so i jumped at the chance.
some shots i took with my tamron 70-210 2.8 - thru a window, on a tripod. about 500 feet to the truck.
no converter, the 1.4, the 2.0, both.
   
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