Add in the 2xTC and for a lens with 2km reach that is abnormally compact.
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We already know something about the MC-20 when used with the 40-150 f/2.8 and 300mm f/4 and, whilst quite good, it degrades image quality as you might expect. It's only using 1/4 of the light. Adding it to the 150-400 f/4.5 TC with its internal teleconverter already deployed sounds over-ambitious to me.
And what are you going to do with a 1000mm f/11 lens on micro four-thirds? Teleconverters aren't generally useful for photographing distant subjects (apart from astro) because atmospheric effects are usually dominant. i.e. you don't see any improvement in image quality over using a shorter lens because the light received by the lens is already distorted by atmospheric effects. You normally use a teleconverter to help fill the frame with a closer subject. You just spread the same amount of light as a crop over more pixels. More pixels can be beneficial but only when there is sufficent light (corresponding to a reasoble Iso value). That's what more "reach" means. So what subject can you shoot at 1000mm f/11 that couldn't be done better another way? Certainly not small, macro subjects. Certainly not bird in flight. Probably not moving subjects. Probably not indoors... Maybe something quite small, not moving, fairly close but where you can't get any closer. Um...
And bear in mind that if you zoom out below 337.5mm (bare lens), you will actually be collecting less light than the 300mm f/4. So probably don't do that either.
To me, the bare lens (400mm f/4.5) makes sense. Internal TC (500mm f/5.6) makes sense. MC-14 alone (560mm f/6.3) makes sense. At a pinch, internal and MC-14 (700mm f/8) might be usable occasionally but I doubt it. And that's about it. YMMV.
