Will the Mavic benefit for having support for the faster uhs-II
mem card availible ?

Meaning if it can be faster write speed maybe DJI can solve flickering video encoder problem , or increase image quality or image write Times' to sd card !

What do you think of this ?

I have been using a UHS-3 for some time and there is not difference in the video.  Still eventually breaks up and crashes the app.

Probably not, they'd need to add a different SD card port.  The newer faster cards have a second row of contacts above the first.

what type of cards can we use? i have a class 10 256GB Sandisk but i haven't put it in yet

No, the Mavic only support UHS-1 Maximum, so it would not be necessary getting a UHS-3 I originally have problems with my 16GB included memory card so brought a new one, I did a lot of research and would say its the best and fastest compatible with the Mavic  http://amzn.to/2v1UULc

We do not recommend using UHS-II memory card, it might cause the recognizing issue due to the writing speed.
We suggest using SD card with Class 10/UHS-1 and above and writing speed with 60Mbps. Please kindly choose original SD cards from the SD card companies, which can guarantee the full test of its stability and compatibility.

Ok thanks for the information about this.

I did notice that images where left out when mission scanning and thought it was related to write speed not performing well

Check this site out. https://cam-do.com/pages/sd-card-speed-tester
Note: you need a usb3 adapter or built in sd card reader. I also use H2testw to get the actual performance of your card

16GB Sandisk Extreme (3)
(Mavic Pro)
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 67.1 MByte/s
Reading speed: 85.4 MByte/s
H2testw v1.4
===========================
32GB Sandisk Extreme Plus (1)
(phantom 2+)
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 56.7 MByte/s
Reading speed: 84.0 MByte/s
H2testw v1.4

================
32GB Sandisk Extreme (3)
(New)
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 51.4 MByte/s
Reading speed: 65.0 MByte/s
H2testw v1.4
=============
32GB Sandisk Ultra
(Samsung Galaxy Note 3)
Warning: Only 28851 of 30424 MByte tested.
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 13.2 MByte/s
Reading speed: 43.1 MByte/s
H2testw v1.4