Just wanted to follow up on what Tord says with his nice personal summary.
I came to a different conclusion because I have different priorities so I thought I'd share them here, but this is not to say that Tord was wrong. Oh, and I am posting this based on his comments and my use without the MC-14 (I get mine in a couple of weeks).
Anyway, for me the speedy autofocus is crucial for photographing smaller birds as they zip around in the trees or bushes. The thought of 1s seek time is an immediate no-go. It's certainly an expensive luxury to get the speed, but if you can afford it then it is nice. I hope that the speed remains with the MC-14, but if not at least I can remove it. Also, the ability to get f4 is really, really nice in the jungle, as is weather sealing.
There's also been lots of talk about the ability to handhold at 1/10s or so, and my immediate reaction is 'so what?', since birds move too much for that anyway. This could be handy sometimes (I still remember handholding the Canon 400 f5.6 for a 0.6s photo of kingfisher in the dark. Keeper rate was atrocious but I got a couple!), but what I have noticed is that after walking in the heat for a while, I have been able to get sharper photos at 1/100s etc without having to pause to catch my breath or steady myself. I've therefore been more confident to keep my ISO down a bit at 800.
Bottom line is that there has never been a better camera and lens line-up for amateur birders I reckon. With great value offerings from Canon and the new 200-500mm from Nikon, and this 300mm from Olympus there is a range of truly hand-holdable camera set-ups.
I'm very happy with my EM-1 and 300 f4 Pro, and I am looking forward to the MC-14. A portable, comfortable, hand-holdable magnification equivalent to 840mm that I don't have to worry about if it rains. It's my dream lens.
Attached pics are a couple in good light and one in the dark
This piculet was shot at ISO 800, f4 1/400s. It was underexposed so the background didn't blow out and then selectively increased the exposure of the bird. Not perfect, but I doubt it would have been this good with my previous equipment.