EC-20 AF with E-M5
And the focusing was very veeeeeeeerrrrryyyyyyyyy slow. I think that and your problems relate to slowish max F-stop (F7) and to the fact that either EC20 nor 50-200 SWD are not optimised for CDAF. The moter system (SWD) may also have something to do with your problem, although the 12-60mm lens works very fine alone (but is a bit slowish).
Speed is definitely not the issue, nor is F-stop. The lenses focus accurately without EC-20. Yes, it's slow, but it should still be accurate on still subjects.
Most of the AF-problems I had were because of the slowiness of the AF - the subject just had too much time to move. I photographed some swimming whooper swans.
With MF the camera wont use the AF circuit, se the response is much faster. Also the enlarged view helps for exact focus.
I have not used the 50-200mm lens too much with EP3 because of the size/balance/ergonomics/slow AF problems. But the results were very good when I used that combo IQ-wise. I think the handling must be a lot better with EM5 and the grips - not tried yet.
The HLD-6 is a must-have for using large lenses on OM-D. It rocks!
'The best camera in the world is the one you have with you when you need it'