RubanCam

Oops, you are right about the crop. I was thinking the M4/3 crop as 2, not the crop factor of 1.9 from the P4K (based on relative sensor width). Super35 is normally a 1.39 crop, not the APS-C size of 1.5-1.6, so the P4K with a speedbooster is 1.9x0.71 = 1.349 is slightly wider than Super35 and the 0.64x booster is even wider.
As to image quality, I am very curious about this as well. I am going to be comparing the P6K vs. the P4K with my 0.71x speedbooster next week. I suspect that there will be little to no difference. I also think that the P4K + booster will be better in low light. I can't imagine that the P6K is one stop better than the P4K, given that the P6K has smaller photosites. If BM had used a "true" Super35 sized sensor (~1.39 crop), then maybe the P6K would be >=, but they used an APS-C sized sensor (which they are calling Super35...)

The field of view will bit a bit wider with a P4K + Speedbooster. The crop with a P4K + Speedbooster = 1.28 (0.64x) or 1.42 (0.71x) vs. a standard super35 crop of 1.5-1.6.
The P6K sensor is larger and is able to capture 6K raw images and some faster frame rates than the P4K. Most everything else is very similar / same. Also, I have seen comments to the effect that a 6K image downsampled to 4K makes up for the Bayer pattern and will produce a "true" 4K image. So, theoretically the P6K image may be a bit better than the P4K.
Plus, you get bragging rights, since many people bash the P4K because it *only* has a M4/3 sensor. They won't be able to say that about the P6K (/sarc) 

This camera is going to lead the FF hybrid market with the Raw out put to Atomos ninja V this summer . 
I use GH5 with Leica branded lens but would like to have more flexibility in colour grading. I was going to buy a  BMPCC 4k but after hearing the Nikon raw out to Prores RAW, I am seriously considering  to buy this camera.  The colour science seems better than Lumix S1.

missing my 4K 60 though. 

Actually it's a still raw file, not a video file. What they were talking about was using burst mode to take many raw stills and then use them to create a video. 
Anyway, is it really 24fps?

GH4 also allows recording 10 bit 4:2:2 from HDMI. It never really got people pointing a real "wow" difference vs the internal 8 bit. So its for Panasonic to show us a real improvement in GH5 picture quality vs the GH4.

New info
I am told they are indeed using a multi-aspect sensor.
It measures 18.9mm x 13mm in approx. 3:2 aspect ratio
20MP stills but the total resolution of the chip is 24MP
24MP 3:2 is approximately 6000 x 4000 (just like the Sony A6300 in 3:2 stills mode).
Then the 6K photo mode is 3:2 with full sensor readout up to 30fps burst.
The normal stills mode is a 20MP 4:3 crop of the sensor. This is why 43rumours say 20MP sensor.
The 4K video is a 16:9 crop of the sensor using the full 6K width but downsampling on the chip.
Absolutely take this is a rumour until confirmed.
But it makes total sense to me... There is *no other way* of getting 6K photos from a 20MP 4:3 sensor like that featured on the GX8.

i think it will be technically difficult task given the clearance between the sensor and the mount. If you go more narrower, the Speedbooster will protrude and hit the sensor.  BMPCC 0.58X barely fits and you may also get soft corners and vignetting
Yes it does, I shoot pictures and occasional 1080P video.  Only issue i am having is with this setup is when a wide angle lens is used, the corners are slightly softer ( out of focus) if you focused in the middle of the frame. No issue with 35mm and up.  4K Crop on GH4 is actually 2.3 not 2.2. The plus side is 2 stops light gain, FF coverage and the FF Bokeh.

But does the 0.58X Speed Booster work with the 2x crop instead of 2.2x?

wouldn't it be amazing if panasonic worked together with metabones to make a 0.55x speedbooster so that they could hit the full frame audience 

The problem is that video people tied themselves into Canon based on the notion that Canon would start adding video features and make video centric DSLRs... Unfortuntly, there was zero evidence of this happening... So they kinda have themselves to blame.

Add that to the petition...
 

Now this leaves  Sony is the only camera maker Gives FF 4K video with a versatile lens mount and hopefully their next iterations would have better video quality. 
As a video shooter, i Switched to Panasonic from Canon when GH2 realesed and never regretted.  My hope is that GH5 would have no crop factor in 4K mode. 

Most museums do not allow tripods, monopods or gimbals (or selfie sticks). So camera/lens stabilization is essential. A museum video captures the experience of being at a museum - you see the exhibits, the space itself, and the people. Getting the colors right, for the art, is also essential. So, a good test for a camera. One oddity here is that the space was sunlit from above (skylights), and the sun was moving in and out of clouds - so the light changes in some shots. It's not the camera. Part of the true experience.
 

GH4 C4K, BMPCC Speedbooster, Nikon 105 2.5 Ai @ F4.   
 

4K60 and 1080/120FPS
more realistically

Yes it is the bad pre-amps, general audio quality via the ext. mic socket. The lack of XLR adapter doesn't help either and in-camera audio features are only basic. Would have liked to have seen an 'audio scene' setting like the Canon XC10, an XLR adapter like the GH4 gets via the YAGH and high quality internal circuits / pre-amps.
Real shame the NX2 won't ever arrive.
I still love my NX1, it is fantastic ergonomically and a bargain for the image it provides.

Highly doubt it.

The GH series of cameras are just fun to use.   The plucky GH2's image still excites me all these years later.  The GH4 is the camera that really made the GH a professional tool.  Panasonic listens to users and gives us cool things.  To me that deserves some loyalty.     I'm looking forward to the GH5, but I'm not convinced I'll buy it until after some reviews come in. 
I'd like to see them offer something neat like programmable rack focus.   The post focus feature they have now is halfway there, and it would be a fun feature to play with and perfect. I'd use it all the time. 

But they have. The G7 was a huge step forward in low light performance. 

Not as good as the Sony, but a damn good performance nonetheless.
 

Have been the GH system owner since GH2, had GH3 and Using GH4 with BMPCC speedbooster , Nikon prime  AIS lenses  plus some Leica lumix lenses and now eargarly waiting for the GH5.  Bought an A7sII  out of hype and sold it after 3 months since it did not do anything better for my shooting needs.

GH5 Would be an awesome camera to me if 
 The sensor is bigger than M43 like the one JVC use.
10bit 4:2:2 internal processing and HDMI out with like the GH4.
Better internal codec and colour science with more info for post work.  
Manual  focus Zoom  in  option while recording.
Better slow-motion options.
Slightly bigger body than GH4 and Do not make it smaller than GH4.
One  or two extra stops of light sensitivity over GH4
 
Panasonic can also add dual base ISOs and IBIS  if they feel like giving us more
 
The only thing that is going to make the GH5 popular like GH4 is the sensor size. not 6K.

Agree completely about the shiny spec sheets. Currently shooting with A6000, RX10 II and A7R II, all of which fall short of the hype in real working conditions. Especially the rolling shutter of the A7R II...

A brand new a6000 is 60-70% of its original price today. Like all Sonys, if I want an a6300 I will pay 70% just a few weeks after release.
Either way, I would get the D750 or a Blackmagic.
The a6300 is a camera I would never buy without testing. Sony has lost my trust completly when it comes to shiny spec sheets.
Might be a killer camera but I wont risk it untill I know.

I wonder if Sony can implement the better codec as a firmware upgrade to A7R II / A7S II.