Opticallimits

OM-D E-M1X
https://www.olympus.com.au/Products/Inte...X/Overview

M.Zuiko ED 150-400mm f/4.5 TC1.25x IS PRO (Ok, this one hasn't landed but will land in 2020)
https://www.olympus.com.au/Learn-Explore...0-400mm-F4

f4.5 x2 = f9, +2x TC = f18.

Luckily MFT is exempt from the diffraction of light.

You forgot the internal 1.25 teleconverter, so we are at f/22.5 :-)
With an ND filter simulation built into the camera, aperture blades are overvalued.

Well, a "native" (x1.25=) 187-500mm f/5.6 = FF equiv 375-1000mm f/11 is not that bad - even from a diffraction perspective.
It may be interesting whether the HD mode is still working at 1000mm hand-held ;-)
But with a 7.5 EV total efficiency, it should be quite cool.

Wonder what the price is going to be like?

If you look at Fuji, they charge for their 200/2 the same as Nikon or Canon for theirs. Which are FF.

So, I think it will be not too far away from a 200-400/4 (older versions)