It varies, but generally speaking, across all games and cards and hardware running underneath them...a 50% boost in performance. Of course, SLI/Crossfire configurations tend to have issues like higher framerate stutter or sometimes artifacting.
Unless you're dying for FPS it usually makes sense to wait until replacing it with a single GPU for the same price or less than doubling down will yield at least that performance boost. In other words, when there is a single GPU that costs the same or less than buying a new R9 280, and boosts your performance beyond what 2x290's will yield, then you finally have waited long enough that you should start thinking about an upgrade. Until then it's generally just not worth it to upgrade your hardware unless you have a specific task/target you greatly desire that your current hardware is insufficient to meet.
Here is a Tweaktown article specifically benchmarking the R9 280 vs. Crossfired 280's (I linked 3DMark11, but there are specific games benchmarked if you check the drop-down menus at the bottom to go to the next pages in the article):
http://www.tweaktown.com/reviews/66...aming-oc-in-crossfire-oced-review/index4.html
The guys without issues at Guru3D are saying the Mantle scaling is fantastic for
DA: Inquisition, though. Saying 95%. Meaning you effectively double your framerate. Usually that's just a theoretical high seen- at best- only in synthetic benchmarks:
http://forums.guru3d.com/showthread.php?t=394922
However, read through. Some are getting terrible stutter, and some are getting constant crashes. Maybe they just don't know who to perform advanced troubleshoots, but...these are the types of guys who post at tech forums. I mean, c'mon. There's a reason that, generally speaking, I recommend single-GPU setups. Just avoid all this mess.
Download FRAPs to find out what your FPS is. Very simple to run.
