after having read through this thread and lots of others on different forums regarding this lens ,i do think that a lot of the alleged problems are simply and purely user error ,and its going to take a while to sort out the the wheres and whys .
(1) quiet a few mentions of multi point /mixed focus points at extreme range
the answer ,is you should really be in single point ,servo mode a/f .multi point a/f will be looking all over the sky for a target .also if your camera has tracking speed enabled try moving it to fast rather than the normal slow .
(2) lots of references to camera/lenses on tripods with the i.s switched on .
the answer ,if your lens is tripod mounted switch off i.s as it will be working against you constantly ,try to pre-focus your lens somewhere in the region that you think your target will be and if you have back button a/f keep it locked in this area to save the lens hunting its full length .
(3) its not a L lens and comparing it to one is like comparing a mini to a ferrari ,its a mid range long reach zoom from a third party manufacturer .a lot of these reports have put me off contemplating getting one of these lenses ,which in all honesty after looking at comments in depth isn't really fair on the lens ,trouble is no experienced long lens wildlife shooter who uses a ,for the sake of argument a 1D mkiv and a canon 600mm L lens is going to test this lens out and give a fair appraisal as they will be biased ,and putting it in the hands of inexperienced users will lead to what we are reading here .
changed systems to MFT olympus omd1-mkii with ver 3.01 firmware , panasonic /leica 100-400 , panasonic G80 ,panasonic 12-60 mm. olympus 30mm macro (on order) various convertors to m42 and nikon mount lenses