A fair question, Balinju!
I'm going to refer you to
these results, for my reply.
If you go down to Bill's test results, you'll find (as of today), 9 blocks, rated with a c/w between 0.19 and 0.26. That's degrees Celsius per Watt.
So for Cathar's block (0.19), assuming a 70 Watt load, the temp difference is 13.3 degrees C. This is, in the case of this block, a temp difference between CPU and water, at a flow rate of 1 gpm. (at 2.5 gpm, you can actually get close to 0.17 C/W). This is the best block known to the entire watercooling community, and I mean worldwide!
To answer your question, I was referring to an idle temp difference.