Nikon 200-500mm vs Canon 100-400mm II

[Aglet]

1. The diffraction limited aperture on M4/3 with a 20 mpx sensor is 5.9, slightly below the widest aperture of f/6.3 for the lens at 400mm. In practice, the lens has hardly any more real reach, if none at all, than a 400mm on a 7DII or 80D or 5DS, but has a very restricted field of view equivalent to an 800mm on FF, which makes it very difficult to use for birds in flight or even finding them when sitting still in a tree, as well as the drawback of being mirrorless.

that diffraction-limitation is not too big a deal at these still "normal" apertures, the output is still very good.
for BIF ML may still be a drawback in some shooting scenarios. I don't do this sort of thing enough to appreciate all there is to it.

2. I use the camera and lens as a spotting scope, and this would be more difficult with the GX8 at 400mm, as well as running down the battery very quickly.

but not so difficult at 100mm and them zooming in with a stabilized display to assist.
battery life - agreed.

There are one or two other problems. I think that you would be restricted to using the lens with Panasonic because the IS on the lens has to be coupled with that on the sensor. Despite being mirrorless, the camera has shutter shock and so has to be used with the electronic shutter at lower speeds.

we don't yet know if Oly's 5-axis system, designed to work in tandem with the OIS on their new 300mm lens, is also set to work with the 100-400 Pany. It might, at least, provide roll correction using the IBIS.

If you are already into Panasonic, the 100-400mm would be an excellent acquisition and would provide many excellent shots. If you have Canon gear already, you would be sacrificing flexibility and a superior system, but it might be worth it if you didn't want to lug around 2.5 - 3kg.

I'm considering the 100-400 Pany as an add-on to my EM10, ATM. I may get the EM1 or its v2 eventually, in which case I'd expect a bit better overall performance, but still get the reach with my current MFT body.
OTOH, the 100-400 Fuji on my XT1/10 bodies should work quite decently as well. The new XT2 may have the same or better improvements as the X Pro 2 which could up overall performance a bit more.

I'm tempted to buy a bazooka zoom for my FF Nikon but the uses I generally have for long lenses don't lend themselves to large prints so the overall IQ of the Fuji, or even the MFT systems, would suffice for me.

It's nice to have choices, even if there isn't one all-around perfect match.