I just received a copy of the Japanese Johnny Mnemonic / JM BD from Amazon.co.jp so I'll share the details. First, the disc is labeled as region A/B/C so cheers to that. Though much to my disappointment the version of the film in HD is the US version instead of the Japanese cut. However, the Japanese cut is included on the disc in standard definition as a bonus. The other extras (all in SD) are: a making-of EPK fluff piece, some behind the scenes production footage, a handful of very short cast & crew interviews (including William Gibson), the music video for Stabbing Westwards's "Nothing" and a trailer. Menus are all in Japanese, but with the basic layout there's little trouble with navigation.
Moving on to the technical details, the video encoding is MPEG4-AVC and I'm happy to say that I did not notice any significant compression artifacts, EE or DNR. The picture was sharp throughout and detail was quite good, but the one negative is that there were many instances of specks and dirt. The CG scenes do look especially good and overall the PQ feels as close to seeing the movie in the theater as I can recall. With all the DNR madness of late I have to say that seeing proper film grain again pleases me to no end. Audio on the HD version is TrueHD, but as I lack a compatible receiver I can only comment on the DD shadow and that does sound fine. The BD is a definite improvement on the Superbit release, not to mention the original R1 DVD.
The Japanese cut is presented in 16x9 using MPEG2 with 2.1 DD in both English and Japanese. The picture is very soft and the black level is a bit too high so shadow detail is lacking. Subs are Japanese only and just like the Japanese DVD the additional scenes with Japanese dialogue have no English subs. The quality of this version is slightly higher than the original R1 DVD, but not nearly as good as the Superbit.
With regard to the narrative differences between the two cuts there are many sites that have gone into great detail so I won't waste people's time repeating every alteration here. Having already been a fan of the US version I found the changes in the Japanese cut made that version considerably more enjoyable. Though if you didn't like the movie I'm not sure the Japanese cut will make you a convert.
Bottom line, the Japanese Blu-ray is the best release of Johnny Mnemonic available right now. Not only do you get a nice version of the US cut in HD, but you also get the Japanese cut as a bonus. Having personally tried to acquire a copy of the Japanese DVD to no avail I'm pleased with my purchase despite the price.