Well, you are going to need as much help you can get to keep the camera steady, because I would never use more than ISO1600 on any m43 sensor, and I really dislike the splotchy noise profile on the EM5 and EM1 even at low ISO. I don't get why people say ISO3200 is fine on the camera, but I suppose I'm just being picky. I do not however expect technology to improve much, since the technology has plateau somewhat. Maybe Organic sensors might allow them to squeeze out a stop or so but I won't hold my breath.
So if there is one good reason that Olympus must go to a larger sensor, is because a larger sensor will always most certainly give a cleaner image. But then again, given the issues the company has, I don't think they have the ability to push through such a project.
When using any 400mm lens, to get the best image you should be using a support. I use a monopod. Trying to get a good image on a 400mm lens without one is hard, IS/VR or IBIS. I don't think you'll see too many people using the three pound Sony without support.
You don't expect technology to improve much? Over what time frame? ten years ago a flip phone cell phone that couldn't display an entire 140 character text message was cutting edge. Today cell phones that play back 720p movies and out-shoot p&s cameras from 10 years ago are the norm.
The EM-1 has better IQ at the same ISO than an first gen 5d (released 8 years ago). Check noise images here
http://www.dpreview.com/reviews/canoneos5d/21
and
http://www.dpreview.com/reviews/olympus-om-d-e-m1/18
playing with the second link, I'd say the E-M1 is actually about a stop
better than the original, and 8 year older, 5d.
And the m43 sensor has a trick up its sleeve to get away with using a lower ISO than FF: DoF. If I use a FF 50mm 1.4 lens at 6 feet for a portrait, my Dof is 4". Using a m43 25mm 1.4, my DoF is 8" - the same as using the FF at 2.8. If I want most of a person's head in focus, I can shoot the FF at f2.8 ISO 6400, or I can shoot m43 at f1.4 ISO 1600.
It isn't magic bullet, there are times when a razor thin DoF is acceptable and the older FF camera wins out. And of course, keeping to the same technological generation, the FF takes better pictures, period.
I think the plateau effect is due to so many minor revisions being released each year. True the Rebel T5i is not practically better than the T3i, but they are only three years apart. When you look at cameras that are only updated once every five years (like the 5D) you see big jumps in IQ with each generation.
You're right about Olympus not being in a position to push a FF camera, but I think that extends well beyond just Olympus. I don't think the market as a whole is ready to embrace a fourth mainstream FF option at the price point FF is going for. If I could have bought a FF camera for E-M1 money, I probably would have. But right now, due to either production costs, or simple need to maintain margins, no one is releasing a FF camera that costs what even the most expensive m43 camera costs. And now that I've lived with the E-M1, I'm less and less interested in the trade offs that come with a FF system.
DNG - thanks for that picture, it conveys what I said much much better - what's that old saying about pictures and thousand words yadda yadda...