I regularly stick my cheap [ish] 2x on my Nikkor 80-200mm f2.8 and have no problems with AF [this being the old push/pull lens is'nt that fast to AF but the 2x does'nt really make much difference]. My reasoning for swapping my Sigma 135-400 for this set up was the advantage of having the option of f2.8 - without the T/C fitted - for the winter months. If you're going to be using the converter for most of the time then you might as well go for a longer lens in the first place. Not quite the same options with Nikon as you have with Canon but there are still several sigmas to consider. I did stick a 1.4x on a 70-300mm when I had a Canon 10D but to be honest, the extra was'nt really worth the iffy AF.
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