They stated in their last video/ a forum post that AMD found the issue and are working on getting a fix sent out and it should be ready with their next driver update. Was an AMD issue the entire time that they just needed to find and fix.
That's not the entire truth. It's an issue with the game engine made worse by the current drivers. Everyone has massive slowdown in Supercruise, but if you're on Nvidia or the right driver it rarely drops under 60 FPS .
Also 15.12 out and no fix . Hope they get a beta driver out soon