Here is a translation posted on another forum from someone who speaks Japanese.
1. The 6-stop equivalent of 5-axis image stabilization is only possible on E-M1 and E-M5 MkII (with the firmware 4.0 and 2.0 and later, respectively). On other Olympus bodies, you can only select "either" in-lens or in-body IS. On Panasonic bodies, you can only use the in-lens IS. (I'm not sure about the compatibility with GX7 and GX8)
2. The tripod colar has an integrated Srca-Swiss compatible foot.
3. The decoration ring to replace the tripod colar accompanies the package. I don't like the idea of "decoration rings" incorporated by both Oly and Panny, though. Anyway, the removal or tripod colar reduces the lens by 200g.
4. The lens weighs 1.475g (with the colar) and 1,270g (without the colar), which is roughly the same as the non-PF AF-S Nikkor 300/4.0.
5. The lens is superior to the Zuiko 300/2.8 for the 4/3 mount stopped down to f4.0, in terms of the contrast, the resolution and the image quality into the peripheral areas, according to Olympus.
6. The sample images of the part of Tokyo Sky Tree are shot with Oly E-M1 and Panny GX8. The order is (from top to bottom): E-M1 images: the center with the lens only, the top right corner with the lens only, the center with the MC-14 teleconverter, the top right corner with the MC-14 and GX8 images (the same order). Bare in mind that the photographer suspects the influence of the atmospheric haze may affect the contrast especially when the teleconverter is attached..
7. The blue patches of the sky are the test images for the light fall-off. The three patches below are the results with MC-14.
8. AF is faster and more accurate on E-M1, thanks to its phase-detect photosites. According to the general impression of the tester, 70% of the images shot with E-M1 was of 80-90 point (out of 100) quality, whereas those shot with GX1 was of better than 95 point quality when focus was spot on, but the success rate was as low as 30%.
9. The EVF on GX8 make it easier to chase the subject than that of E-M1. Also, the "Touch Pad" function on GX8 made easier to change the AF point around with his tumb on the touch-sensitive LCD by keeping looking intot the EVF.
10. The tester recommends the "Dot SIte" EE-1 to get the subject back into the frame when you lose it. He uses the additional brackets to place EE-1 so that you can keep looking into the EVF with the right eye while looking in to EE-1 with the left eye. The sample images of flying seagulls and a flying butterfly were shot this way.
You should be able to see the shooting data (shutter speed, aperture, exposure comp., and ISO). The first batch was shot with E-M1 and the latter batch, with GX8.