Have you tried the new 18.9.1? This title runs well for me on my RX 580 and has since the game launched. There have been a couple drivers that messed with free sync and seemed to stutter at times. This latest driver is flawless for me. I realize it may be different on your R9 but this may be a model issue not an AMD issue as it works great for me on my RX 580. I will try it on my R9 380X and see if it is an issue there and report back if it is. If you have not done a clean install of your driver in a while that may help.