If you make that move it wont be for performance, it would be for eliminating any crossfire issues you may have.
Check the different resolutions and the overall summary % differences in the review below. The 5970 is 6% slower than the 7970 and two 5870's in crossfire is slightly faster than the 5970 if I recall correctly (its usually the case due to the clocks not needing to be scaled back). So your performance will be very similar other than in games that do not work well in crossfire.
Let me put it this way, two 5870s in crossfire is faster than my 580 in most situations, and I wouldnt go to a 7970 from my 580. So like I said, it all depends on how happy you are with the functionality of crossfire. Hope this helps.http://www.techpowerup.com/reviews/AMD/HD_7970/28.html