The frame pacing issue has been covered at our website for almost a year now. It stems from the original "microstutter" problem which dates back over a year before we could quantify it. We like to use the term "Frame Rating" to denote the testing methodology we now use for our GPU tests.
AMD fared worse at these tests than NVIDIA (although even they had some problems in certain configurations). They have dedicated a lot of man-hours to the problem resulting in a driver updates for certain scenarios. Crossfire while utilizing Eyefinity or 4K MST was one area they did not focus on. The issue has been addressed in Hawaii and AMD asserted that previous cards will get a software fix soon.
The good news is that we have just received word from AMD that they plan on releasing a beta driver for Southern Islands and earlier GPUs (AMD believes it should work for anything that's not "legacy"). As usual, until it ships anything could change, but it looks good for now.
The beta "frame pacing" driver addressing Crossfire with 4K and Eyefinity, for supported HD-series and Southern Islands-based Rx cards, is expected to be public sometime in January.