The Argonne information leads me to believe the benefit comes from the conductivity of copper, but it's a little hard to believe. Here's what I'm thinking and I an open to input.
There are a few ways to improve heat transfer. Looking strictly at convection to a fluid and transport of energy by the fluid having a delta-T, you must look at convective efficiency and specific heat/mass flow rate.
I think we can rule out a benefit in specific heat/mass flow rate. Copper has a specific heat of 385 joules/kg-°C compared to water's ~4100 joules/kg-°C. Granted, copper is more dense to the tune of almost 9:1, but this gives the edge to water as 9*385<4100. The other thing involved is overall flowrate since specific heat * flowrate * delta-T equals energy, but I can't see these miniscule bits of copper lowering the fluid's viscosity. Without lower viscosity you will not see higher flow.
So how about convective efficiency? A large part of convective resistance is the stagnant layer most engineering types call the boundary layer. In the boundary layer, fluid suffers from a varying degree of stagnation. At the fluid/solid interface there is literally no fluid motion. As you get farther from the interface, the fluid begins moving faster until you transition to the portion where flow may be considered fully developed.
Because the fluid molecules don't move much in the boundary layer, heat transfer relies mainly on conduction through the fluid (same mechanism that gets it through a solid). Think of it as one molecule having to hand-off the heat to another rather than simply "walking" from point A to point B and taking the heat along on its own. A material's conductivity is a measure of how well the material "hands-off" thermal energy. Here copper rules the roost over water by ~400 vs 0.6 and the lead grows even larger vs antifreeze.
It seems to me that the copper in the boundary layer must serve to boost conduction across the layer substantially.
The other thing that could be happening is a thinning of the boundary layer, but this would also require a drop in viscosity. As I noted above, I don't think this is likely.
Anyway, if my take on this is pointed in the right direction then I still think the benefit for pure water is going to be pretty small. Water already has a good, low viscosity that gives it a large edge over antifreeze. This stuff may (probably would) improve things, but I hunch the gain would be a lot smaller for pure water vs antifreeze and oils. The real kicker in my mind is still whether or not you could close the gap enough for antifreeze to compete head to head with water. No biological/corrosion problems and little to no hit on performance. Sounds good to me.
Comments welcomed.
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