Here is a shot I took a few weeks ago using A7r with metabones adapter, 70-300L at ISO 1600 hand held on a rather dark, very cold (almost -30) day. Fun subject since we don't get Mandarin ducks on the north shore of Lake Ontario very often
Also one from last week this one using the canon 100-400L at 400 ISO also handheld but only at -10 degrees so I was only shaking a little.
So far everything works very well, autofocus is very slow as expected but focus peaking works amazingly well (at least with the 70-300, the 100-400 I have always found changes focus with way too small of a movement to work for me). Can't wait to get out in something other than cold grey, white and beige to play with it with more wide angle lenses.