I'm not sure the bird pics you are showing are doing much to convince me that this can be a viable wildlife setup. The images just don't seem to me that sharp and bright. The best ones I see the night herons. Peter, the oystercatchers look slightly oversharpened to me, particularly apparent in the grasses behind the slightly too contrasty birds.
Here is one I think is acceptably sharp, and it was taken with a 50D and 100-400 lens which is not considered the sharpest tele zoom out there.
I still think the OM-D EM-5 looks like a fabulous travel camera, but to me it doesn't yet look like a fabulous wildlife camera. What I am really trying to figure out is how well it would work as a landscape and panoramic stitching camera.
That's a very nice shot of the little raptor, Marc. You did have the significant advantage of a bright blue day with frontal lighting to brighten it up. Unfortunately all the shots in my albums linked above suffer either from back or severe side-lighting (night herons on pond with 100-300, GH2), or shadowed locations (night herons on the nest) shot at Pt. Lobos, CA, and it's coastal fog, at ISO 400, with 12mp GH1, and the Oly 4/3 50-200 plus 2x TC for 400mm/7.1. These at nominal 5.6 for f/11.2 for the DOF needed. The head ranger there, who shoots wildlife with a 5D/100-400, was quited taken with the quality of the 17x22 prints of the nesting series I donated, which hang in the docent's office.
If there's a better long tele zoom out there than the Canon 100-400, as old as it is, I'm not familiar with it - certainly surpasses Nikon's 80-400 in the tests I've seen. Except maybe the Oly 4/3 90-250/2.8 at $6000 and 7.25#, which has only the 12mp Oly 4/3 E-5 or older 10mp bodies capable of fast AF with it. The Canon 400mm/5.6 prime I used to shoot with has a flatter field and slightly sharper center than the 100-400, but not by a great margin, and unfortunately lacks IS.
Although I enjoy nature photography, I lack the passion to do what's necessary to be great at it - studying the subjects habits and habitat, returning as often as necessary for the optimum lighting, and most importantly, getting close enough with whatever equipment you choose to fill as much of the frame as you wish with whatever equipment you have chosen. I've seen enough quality 300mm 100% crops from the GH2/100-300 to convince me that it's me, not the equipment (except when I refuse to carry a tripod), that's the limiting factor.
An excellent fast-focusing m4/3 300mm/4.0 with dedicated 1.4x TC for 420/5.6, or 400/5.6 - with OIS if Panny - would be a god-send for guys like me who would like more reach for our GH2 or E-M5. Such a lens without IS would tip the balance to the E-M5, for sure.
And short of monster FF sensor bodies, I'm convinced that the fine range of m4/3 prime and zoom quality lenses, along with the outstanding sensor qualities of the E-M5, will go head-to-head with anything out there for landscape photography.