Have to reiterate what Mark says above, I have the Sigma 1.4 tc for my 120-300 and its pin sharp, af speed is fast and accurate when it's used with the lens, wouldn't imagine the canon would be better or worse knowing how good their glass is. I also have the sigma 2x tc but that's very average and seldom gets used now, the canon one would have to be crap for the siggy to edge that one!