Noise and reach are tricky subjects.
Noise: the 4:3 versus 3:2 aspect ratio gets in the way of a clear comparison. But let's say that I cropped to 16:9 on both an m4/3 and FX camera: it's about a two stop differential in noise production because you're outputting at the same size, but you have a bigger capture area (more photons captured, all else equal). With DX versus m4/3 we're close to a stop differential.
Another problem with the "noise" discussions is that they rarely talk about what the dynamic range in the scene is. For a lot of wildlife scenarios (but not all, obviously), we don't need a huge amount of dynamic range, so noise discussions aren't very useful if you're maximizing dynamic range in your exposure. The exception tends to be white bird in sunlight.
Reach: you're at 20mp m4/3 and 800mm. The Z8 with the 180-600mm mounted is at 45mp and 600mm, or effectively 19mp at 900mm when you flip to DX image area. It used to be with the Olympus bodies/lenses, you could build a smaller, lighter kit with considerable reach. With Nikon's recent lighter lens offerings, that's not so clear cut any more, at least if you're comparing what you get at ~20mp. As I noted on my site recently, I just had a full safari kit in a 19L bag with room to spare (two Z8's, 70-180mm f/2.8, 180-600mm f/5.6-6.3). Travels light and compact and takes fine images. I'll be firing up a (delayed) daily blog on that trip soon.
