I have a 380x crossfire system, but I disable crossfire (globally, since profiles are broken) before I play Rocket League (to reduce input lag and the scaling is bad in general). Even with crossfire disabled, and even after restarting the computer, I'm getting very bad stuttering with vsync enabled. Frames will drop when turning the camera quickly or when there is a lot of action on screen.
Previously I had a single 7970 and I could run identical settings and never get dropped frames with vsync enabled. With vsync off I actually get higher framerates with the 380x . So something fishy is going on here when I enable vsync. Maybe crossfire frame-pacing is still enabled even though crossfire itself is disabled?
Drivers are 18.104.22.168, and I'm playing on an 85hz monitor (vsync needs to be on point with a refresh that fast)