Well, we are talking about a constant 5.0GHz (as opposed to 5.0 under load, and 1.01v at idle, with voltage offset), right? How long do you need to run at 100% load before you know the temps have leveled off, or do they increase in stages, with some plateaus in between? I had my 2500K at 5.0 for about 20min (yeah, I know, not enough time to test for stability or anything) on Prime95, on a Hyper212+. The temps were hovering around 82-83, but I felt that they definitely would have kept going into the 90's, a situation that I felt was unsustainable (duh). Who knows if the temps would have stabilized below the Tjmax threshold on that air cooler. I think with a good water system, maybe. That was pumping 1.45v, I think. Maybe after I've got my use out of this processor, after several years, I might see what she can do.
That was all under stress testing though. Real-time daily use wouldn't require all 4 cores at 100% except rare occurrences. So, maybe you could set it at whatever voltage leads to a stable 5.0GHz, 24/7, because the load on the processor won't be 100% ALL the time, this should keep the temps at least at a manageable level even on an air cooler (though probably high enough to shorten the lifespan of the chip).