So there is a weight saving using M43 gear, although the big white Olympus 150-400mm does weigh over four pounds and the OM-D M1X weighs 2.2 pounds.
One of the things that soured me a bit about the m4/3rds system has been this whole "let's make things bigger" approach that Olympus has been on, producing over-engineered products (i'm especially looking at the f1.2 primes and E-M1X) that go against the USP of the system.
Luckily they've been getting their act togheter with the 8-25mm f4 and the 12-45mm f4 lenses which are brilliant and rather portable.
If the choice is between the Olympus 300 f4 and the Panasonic/Leica 200 f2.8, I would choose the 300 in a heartbeat.
Generally speaking, the choice isn't as clear-cut as that.
First off, the PL 200 f2.8 usually comes with the 1.4x TC in the box as well, which will give you a 280mm f4, getting you nearly the same reach as the 300mm f4 out of the box (and if the pundits are to be believed, the PL with TC is close as to make no difference in image quality).
While you can get the 2x TC for the PL and reach 400mm f5.6 (same aperture and nearly the same reach as the Olympus 300mm with 1.4x TC), you can get the 2x TC for the Olympus 300mm and reach a (frankly silly) 1200mm equivalent-FOV that is still hand-holdable.
Secondly there is the issue of AF and Dual-Stabilisation. If you shoot Panasonic, you'll want the PL 200mm for their special contrast detect DfD AF and Dual IS. Also for Olympus you'll want the 300mm f4 if you want to have Dual IS and some special functions (ProCapture High and Focus Stacking work only with certain Olympus lenses IIRC).
Finally there is the budget issue... While brand new the PL 200mm is more expensive than the Olympus 300mm f4, I've seen used copies of the 200mm f2.8 and TC14x in excellent state go for around 1200 euros on MBP while Olympus 300mm f4 stays at about 1800 euros. And at that priced for one, the PL 200mm f2.8 is a bargain of a lens

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It seems to me as the OP that yes you can use 4/3 but it needs a 2.8 or F4 lens to avoid the noise problems at higher iso.
As a general rule of thumb, yes. You want the fastest glass to keep your ISO as low as possible. Plus that, by f5.6 you are entering difraction teritory for m4/3.
Now, with modern noise reduction software, like Dxo PureRAW, I am quite confortable shooting m4/3 at ISO3200 and even ISO6400 if the light is good (not flat, dull light).
And you can always use the extremly capable IS systems to keep you shutter speed low when the subject is relatively static.
That may lead to some silly situations like I got into a while back, when shooting grey-headed woodpeckers looking for insects in the grass hand-held at 600mm eq-FOV and 1/200s, where my first 100 shots had the body of the bird tack sharp and the head blurry due to it moving too fast for my shutter speed

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