Man.... I really don't know. Sorry that isn't the answer you're looking to hear.
I've had bad fps 2 times in the last 4 years. ALL of which was fixed by using a different driver.
BUT, I don't use AMD, So I'm kind of shooting blind here.
This really isn't what you're looking for, but have you tried Windows 7?
When the Falls creators update came out, I was 1 more problem away from moving all 12 of my servers back to Windows 7.
Since the spring update and using a slightly older driver, I've been back to my normal FPS performance.
Since AMD and their drivers are something I really don't deal with, another suggestion might be to look on the AMD forums to see if any VR users are having similar issues. I know for nVidia there were ALL KINDS of people pissed off at the falls creators update, because it killed their VR performance (so I linked the 2 user groups of VR and Resolume, as needing the same performance and both taking fps hits together)
Maybe there would be a VR guy on AMD's forums that found a solution (if they're having the same issues?)
Wish there was more I could do. If it was Nvidia, I would say let me team view in and I can play with drivers and settings for you, but I really wouldn't know where to start with AMD.
Sorry I couldn't be of more help