I have to say, I feel more than a little vindicated by this following post about the 120-300 f2.8 OS from Sigma:http://www.canonrumors.com/2014/06/the-saga-of-the-sigma-120-300-f2-8-os-sport/
I had a copy of the previous (non art) version which is the same lens optically and mechanically. I took it to Ireland to photograph Irish sea birds and I was astonished how lame the AF system was compared to my 70-200 f2.8 LIS II, even with a 1.4x TC. It couldn't track a bird in flight, it would lock and then loose AF. It was hesitant and unreliable. I had high expectations of this lens, but I soon sold it once I returned to the UK in disgust.
The focal drop at MFD was appalling too. At MFD, which one would use to try and diffuse the background of a sitting bird, the focal length dropped alarmingly short. Around 240mm by my estimations, far below the 300mm stated and not much different than my 70-200, which doesn't focus breathe. It's a heavy lens for sure and not really worth the bother for the results I couldn't achieve. Where as the 70-200 f2.8 II L and a 1.4x nailed this shots every time and really showed me how bad the Siggi was behaving. These Siggi lenses really are no match for Canon's finest L's. I currently use a 400mm f2.8 LIS and I look back at this Siggi as a tragic error of judgement on my behalf.