About super tele and sensor size

Yesterday I went hiking in the wonderful Parc du Vercors where we met several groups of ibexes and chamoises. Although I had my 100-400L on my 6D, they were pretty far away and I had the same feeling I often have when I do birding: "Gosh, how I wanted some more reach!"
I started wondering whether a crop sensor would be a better option for my wildlife work. At the same time, I am a happy user of a Lumix GX1 converted to IR. So why not a micro-four thirds mirrorless with a 2x crop factor? I could probably get enough cash to buy a body (e.g., olympus om-d em5) and a 70-300mm by selling my 100-400L and this way I would a setup equivalent to a 600mm 5.6. I was just wondering how bad would the noise be at relatively high iso, for instance 1600-6400 iso? and how well does the af tracking work?
I know that probably everyone at some point would like to have more reach, no matter what the maximum focal length they have, and that I would have better results with a 600mm on my 6D or even better on a 1D series or a 7D mk II. But unfortunately one has to be realistic about budget constraints and I know that I will never be able to afford a 10000$ lens, and I am not sure I would enjoy hiking with it. 
I am all in favour of larger sensors, but I also remember that someone once said that large sensor are always better, unless one is focal-length limited. Besides, I kinda like the idea of using different tools for different purposes. Any thoughts and/or experience to share on that?

PS: anyways, I had some keepers yesterday, like this one for instance

What I will say is mirrorless cameras have slower autofocus IMO already, and if you slapping on a 2x converter you are going to now lose 2 stops of performance.  Honestly I would just go with a used older telezoom lens.  Keep an eye on Craigslist, Ebay and even www.keh.com for some good deals on used. 

The focal length & magnification of a lens won't change, only the angle of view when using crop sensor cameras....Thus a crop sensor camera won't allow a lens a further "reach", all it does is "crop" the size of the image projected onto the sensor...

Superman wrote: What I will say is mirrorless cameras have slower autofocus IMO already, and if you slapping on a 2x converter you are going to now lose 2 stops of performance.  Honestly I would just go with a used older telezoom lens.  Keep an eye on Craigslist, Ebay and even www.keh.com for some good deals on used. 

+1

I tried a 2X converter with my PEN, never again.  Barrow a 60D for your lens and see how that goes.

Kelly

Thank you all for your reply but I think you misunderstood what I meant. Never spoke of teleconverters, I was talking about the crop factor. 
What I had in mind was a set up such as GH4 + 100-300, which is equivalent to a 200-600mm in 35mm. Anyone tried that?
Yes, the magnification is the same, but the pixel density is approximately twice as big all other things being equal when compared to an equivalent FoV captured with a full-frame (i.e., what would have taking the shot with a 300mm and crop it to have the same FoV of the FH4). The biggest weakness IMO is the high iso performance: that's always going to be worse than a FF sensor, but how much worse?
And no, apparently autofocus is not that slow ( www.thephoblographer.com/2014/05/27/pana...ound-close-nikon-d4s )

I don't agree with the statement that M43 has slower autofocus than other formats. My EM-10 is just as quick or quicker to focus than my Nikon D200. I will say that focus tracking is where M43 falls short. The noise is pretty good for these sensors.
Here is a handheld ISO 6400 shot with a little noise reduction in Lightroom. The EM-1 is better at tracking in the OLY lineup. I can't speak on other brands much.

I don't have a native lens longer than 150mm for my EM-10 but when I shoot long, I use a Metabones adapter and my Nikon 80-400 lens. The image below was made with that setup. The lion is about 130 feet away if I had to guess. Maybe slightly further.

 

Same setup with the 80-400 at about 50 feet.

Nice images !!! Which 80-400mm was that Randy ?.... ?   The older or the newer one ? ... I was looking to get a longer tele zoom

The older one. It's slow to focus on the Nikon but it can produce some great results in the right situation. Of course it's manual focus on the OLY but the focus peaking helps in that regard. Best results are with it stopped down. On my Nikon, I put it on f-8 and let her rip. The lion was ISO 250 and the orangutan was at ISO 1000. f-stop is a guess using the adapter but DOF preview helps with getting the result you want. I shoot it on Auto ISO on the OLY.

NicoIa wrote: Thank you all for your reply but I think you misunderstood what I meant. Never spoke of teleconverters, I was talking about the crop factor. 
What I had in mind was a set up such as GH4 + 100-300, which is equivalent to a 200-600mm in 35mm. Anyone tried that?
Yes, the magnification is the same, but the pixel density is approximately twice as big all other things being equal when compared to an equivalent FoV captured with a full-frame (i.e., what would have taking the shot with a 300mm and crop it to have the same FoV of the FH4). The biggest weakness IMO is the high iso performance: that's always going to be worse than a FF sensor, but how much worse?
And no, apparently autofocus is not that slow ( www.thephoblographer.com/2014/05/27/pana...ound-close-nikon-d4s )

While yes part of what you say is true. For Nikon at least the increase in MP means you can crop more. But with a high MP cropped sensor you can as well. And since it is cropped already you may gain some benefit. The problem will be with the ISO performance on the upper end. Now you could always go to DP review and use their comparison tool to see how each stacks up side by side. Or even better DxO and use theirs for higher ISO comparison.
When my D300 dies I am not even looking at a crop body as a replacement next time. And I also shoot wild life, that is because I would be able to crop heavily now, as opposed to before. Also the newer bodies allow you to shoot in DX mode even when you have an FX lens attached. So I can still crop the initial image field once if I chose to.

Thank you Shadowfixer, this is exactly what I was looking for. Nice shots!