I've actually had good luck with the 50L on my Canon bodies. To me, it isn't in the league of 'excellent' like the latest (24-70II, 70-200II, 100-400II) for focus but I have even shot sports with the 50L on a 5D3 and been happy.
Within the usual caveats for Sony adapted (decent light, not extreme action) the 50L nails it. The 85L II also nails it, except I get lockup and no-go periodically. I use the MC-11.
The MC-11 with the Sigma 120-300/2.8 Sport seems pretty good. But it's winter, so hard to really test. One thing I really use my Sigma 120-300 for is shooting batters from the outfield fence (with a 2X TC) and since I do this all pre-focused on the 5D3 using the LCD, I really want to try it with the Sony.
forum: Camera Vs. Camera
My issues with the 50L were more due to focus shift and microfocus issues when shooting portraits at fast aperture. Loved the look of the lens, but man was it frustrating to use.
Interested in hearing more of your thoughts with the Sigma 120-300 and MC-11. I have a casual need for super telephoto stuff...mostly my kids sports, and don't want to spend 100-400 GM money.