I think from a practical overclocking potential, you won't find any difference regardless of what their thermal performance is. But if its degrees you're measuring, I would guess VGA thermal performance would be close between the EVGA and Koolance blocks, but the Koolance block may do better at overall cooling the non VGA areas.

BUT...thinking in terms of overall usage in the most common systems..I like the Danger Den block because of the low pressure drop. I suspect if you surveyed the market, you would find that 90% or more users will run combined CPU/GPU loops. It may not have all that surface area, but it probably has a good balance of cooling vs pressure drop. I've always been along the lines of spoiling the CPU as much as possible because it tends to be a bit more critical to cooling. By nature of the constrained slot area to work in, most GPU stock cooling systems tend to be poor in comparison to the CPU, as a result they usually run hotter and are designed to do so.

At least that's the way it used to be. That's my guess anyhow...gonna have to retire this old 8800gtx someday and upgrade..my old system is getting pretty outdated by todays standards..

That's just my guess though, the DD block does appear to be king of low pressure drop. In separate dedicated loops, that's irrelevant(all that matters is thermals), but in a combined loop with the CPU, I personally like having the lower restriction