First off all, a very, very big thank you to all the AMD staff for the new 17.6.1 drivers! The crossfire stuttering in the gameplay is finally fixed, and I can report positive GPU scaling. I can now finally play ME:A in 4K with ultra settings at roughly 35 to 40 fps. Thanks guys!
My presumption is that the fix had something to do with the anti-aliasing? During cutscenes flickering happens, and a huge fps drop, especially on the tempest. This form of flickering looks different from the flickering issues with the previous drivers. Cut scenes with uniform background lighting (i.e. outside / open areas) are less of a problem. It therefor seems a bit related to the CF fix of the 17.6.1 driver.
During most cutscenes it almost looks like a black frame and a rendered frame alternate.
Another issues is that the fix relieving the flickering and stutter during the gameplay did not solve this issue in the galaxy map
GPU: 2x [MSI RADEON RX 480 Armor 8GB OC]
CPU: Ryzen 1700x
RAM: 2x [F4-2800C16D-16GRR]
Motherboard: ASUS Prime X370 pro
PSU: Corsair CX750