I'll have to take a look at the Sigma. I know Canon embeds a lot of AF related functionality into the microprocessors of their lenses, as well as their teleconverters. I've always worried that if I used a non-Canon lens that AF would not work as well or as efficiently.
I read a review
from a few years back of the Siggy 300mm f/2.8 prime, where the reviewer - a knowledgeable and experienced bird guy - stated quite unequivocally that (attached to a Canon body - an old pro film body, but that might not matter) with the Siggy
2x attached, AF was pretty much as quick as the bare lens.
However, upon depressing the shutter button to activate autofocus, I immediately noticed the first real difference; unlike the Canon combination, the Sigma 300/2.8 appears to focus as swiftly and surely with the converters as it does without them! This has always been a trait of the Canon lens/teleconverter combination that annoys me; as soon as I install the matched Canon 1.4X teleconverter on my EF 300/4L lens, autofocusing becomes noticeably slower. Even with a 2x teleconverter in place, yielding a 600mm f/5.6 super-telephoto, the Sigma's autofocus performance remains superb.
On that basis I picked up a used Sigma 2x myself last weekend (the older non EX model - it was cheap enough at £109 that if things didn't go as well as planned I'd be able to offload it again with little or no loss) and tested it out yesterday with my 120-300mm f/2.8 OS and 7D.
While I can't say for sure that it was as fast as the bare lens to AF, it was definitely
as fast as the Siggy 1.4x I've got (which is - I think - a teeny bit faster than my Canon Mk II 1.4x), and with little or no observable IQ hit: in fact sometimes the 2x seemed to be sharper than the 1.4x, but because I shoot handheld and was in and out of all sorts of changeable light I'm not reading too much into that.
Suffice to say though, the Siggy and Siggy TCs work very
well together - I'm really pleased by my experiences so far: and for the avoidance of any doubt, I am obsessively
fussy about IQ.
Even so, my next step will be to bung my Kenko non-reporting
1.5x in between the body and 2x, and see how usable 900mm is!