Yeah, the 511's make the GPU use more power and run about 10C hotter than the others do.
Not sure about just the 280, as it's being stepped up right now. I remember doing the OCCT PSU stress test, which is Linpack for the CPU and a GPU test combined and it showed about 600w from the wall. So we'll assume 80% efficiency, so that would be 480w. Seems to me like it would work with 2 of the GTX 295's...barely.