You'll be fine. The GPU doesn't need a single rail with 30A. It needs a PSU that can output a total of 30A. (more is usually better.) If it can do 576W, then 576W / 12V = 48A total output. This is more then enough for a single 6950.
Seasonic has a habit of doing that. In my wifes computer she has a seasonic built Antec EA500. Two rails, both rated at 17A, 34A total. I have the newer Delta built Antec EA500D in my computer. Two rails, both rated at 22A, 34A total. They just raised the OCP limit in my PSU, otherwise its the same PSU. Always do the math like I did because its the right way to do it. NEVER just add them together. You might get the right answer, but its the exception.