Not trying to derail your question, just curious why you would look to sidegrade from one GTX 580 to another. Even if you get a great overclock, you're going to be at the same general performance level and will probably lose a chunk of money on your current 580 if you sell it.
TH used two of the PNY water cooled units in their last $2400 System Builder Marathon build. Might want to check that out if you haven't already. Keep in mind, they show benchies for stock and then OC- this includes both the CPU and the GPUs OC'ed, so it's tough to nail down just how much the GPU overclocks contribute. Hope that helps a little.