Hi,
Sandisk 256GB UDMA7 160MB/s (1067x) VPG65 card received and I ran a few tests:
1) There's no capacity issue between 5DmkIII and this card. 5DmkIII shows the whole card and when you format it within the camera it makes it perfectly exFAT. Grateful for that.
2) I managed to fulfil the whole card with one .RAW file, did it multiple times with different resolutions formats, frame and crop rates.
3) Recording speed didn't drop below 87MB/s (it depends on what format you use). If it started with 97MB/s it also ended with similar speed after ~48 minutes, so I may say that this card is extremely stable. Unfortunately it also didn't exceed 100MB/s write speed like a hoped a little bit. Tried that with recording some higher level cropped resolutions.
4) Everybody probably knows by now that this time/capacity related defect with buffer was fixed by ML recently and there's no issue with speed drop after 8-9 minutes of constant recording.
5) Camera temperature didn't raise above 68c through the whole recording process. It was lower at the very beginning and received 68c peak after about 13 minutes of recording. Then it stood around 60c - 68c rest of the time.
6) Blackmagic disk speed test on Mac shows constant write speed ~124MB/s and read speed 132MB/s.
7) Successful continues recording formats without frame drops:
1920 x 1080 (16:9), 23,976fps, 24fps, 25fps.
1920 x 508 (2,35:1), 60fps
crop mode: 2240x954 (2,35:1), 23,976fps, 24fps, 25fps.
crop mode: 2048x872 (2,35:1), 60fps

Don't use Kingston FCR-HS3 USB3.0 UDMA7 card reader with this specific card, it doesn't properly support it. It won't allow you to copy all the recorded files from the card, neither write nor format the card. I'm now using Transcend TS-RDF8K (the one that was witness for several Komputerbay 128GB 1000x CF cards death.

not sure what this was card reader fault or faulty KB cards or my iMac or Blackmagic disk speed test. Most probably the extremely unstable Komputerbay 1000x cards because one of them died in my camera once battery unexpectedly ran empty).
This Sandisk 256GB card deserves a new topic, I'll start it as soon as I have a moment.
Happy upcoming X-mas for everybody!