http://www.techpowerup.com/reviews/ASUS/R9_280X_Direct_Cu_II_TOP/26.html
I think that it is so close, that from a strict performance standpoint there is little point in doing it. But there is the added benefit of less power, less heat. Also a single card will will work better in all games, not just ones with crossfire support. That and the 280x has more vram, more vram=more AA, but at 1080p that shouldn't be too much of an issue.
All of that being said if you have 2gb 6970s, Which I assume you do... It's not worth upgrading right now. I have a 2gb 680 and overclock the crap out of it and play all kinds of games on ultra, I will upgrade when a worthy upgrade comes up at a reasonable price. $650-$700 isn't a great deal IMHO for a 290x or a 780ti.