Let's say Non-Ref R290X's come out, and expand that overclocking headroom. Enough to give them a 5200-5400 score. Then let's say higher end Non-Refs like Lightnings can do 5500-5600 or a little more. Non-Ref 780Ti's will be able to do 5800-6000. Then higher end Non-Refs like the Classifieds can do 6200 or more even.
Realistically, AMD is in trouble.
Non-Ref 780's already compete with the 290X and probably even some or most will with Non-Ref 290X's. Non-Ref 290X's are not going to be at $550-600 price range. They will more than likely be $625-700 ($700 for higher end versions). With a 780Ti being @ $700 and Non-Refs being @ $750 even then they are still in trouble. Even if a 780 Ti Classified costs $800 or even Lightnings. Realistically, they will have no more bargain room for the R290X. As in, we have the fastest and most inexpensive cards available. When in reality, you will be able to get an nVidia offering (the 780 Ti) for slightly more money and still be able to outperform the 290X by a large margin (even their best versions).