We have been waiting a long time for this, so in the meantime Tascam came out with a similar product, the DR-10 Chttp://tascam.com/product/dr-10c/
I was hoping that the DARlink would be cheaper, but maybe it has better preamps?
I ran some tests on my a7Rii with and without a battery grip and with a usb battery pack attached.
In 4k Crop Mode with the LCD always extended and NO battery grip the camera did not overheat at all after more than 5 consecutive 30 min videos.
Each battery lasted around one hour, so I switched it every second video.
When using an external USB battery at the same time, the camera ran hotter, but did not overheat for at least 4 X 30min consecutive videos.
The external USB battery I used has a 2A port which is not enough to completely power the camera, but it helped the internal battery power the camera for 4 X 30min videos instead of 2.
When recording in 4K Full Frame mode WITH a third party battery grip attached, the camera ran hotter than in Crop Mode, but again did not overheat for at least 5 consecutive videos.
4K Full Frame + Battery Grip + USB Battery Pack was even hotter, and even though the camera did not say that it overheated, it shut down when the first of the two batteries inside the Grip got depleted after 3 and a half 30min videos (1:45hours).
After re-powering the camera though, it recorded 2 X 30 min videos without issue, so maybe it was a problem with the third party battery grip.
Do these problems happen when using a third party battery grip by any chance? The MEIKE/NEEWER grip in a7r2 seems to trigger random Custom buttons even when in Off mode. But covering the top row of pins on the dummy battery part of the battery grip disables all custom buttons and at least it works ok as a dual battery system.