I recently went on a BIF (birds in flight) shoot with my E-M1,40-150 f2.8 and the 1.4 teleconverter. I couldn't believe how poorly the autofocus performed! I don't think I got one useable shot! The lens kept hunting and by the time any semblance of focus was reached the bird would leave the frame. I may have captured 1 or 2 semi useable shots of of several hundred! I realize that this is a difficult AF situation for any system, but frankly I was very disappointed. I had brought along my ancient Nikon D300 and what should have been a much poorer performing lens (18-200 mm, which is <1/2 the price of the Olympus 40-150!) and it vastly outperformed the E-M1 and reportedly one of Olympus' best pro lenses. I also had quite a bit of difficulty with the E-M1's otherwise excellent EVF with fast moving birds of prey (mostly Sharp Shinned Hawks and Cooper's Hawks). Until this BIF shoot, I had happily phased out my Nikon SLR and thought I'd never look back. I was also looking forward to the new pro 300 mm f4 for BIF work, but now I'm having some second thoughts about completely giving up on my SLR system for super telephoto/action work. I'd love to hear other people's experience with the E-M1 and this 40-150 + TC for fast action like bird photography. Hopefully positive autofocus experiences! Thanks!