Greater surface area with a more complete water contact will work the best. (I know I'm being vague with that answer as there are many variables not considered in what I've said thus far.)

A larger pipe going into a single smaller pipe will give you a pressure increase, a larger pipe going into a set of smaller pipes with a total cross section larger than the pipe you're going from will give you a pressure drop. A 2" pipe (all numbers here forward assume the dimension as the ID of the pipe) has an area of about 3.14 square inches (sq.in). A 1/2" pipe has an area of 0.196 sq.in, so to have no pressure increase, you would need approx. 17 1/2" pipes to flow the same volume at the same pressure.

Without some time and reference materials (that are at work), I couldn't even venture a guess at the thermal transfer you could hope for out of this system, but the idea is sound and you have the basic principals already it appears. Someone who del as with thermodynamics on a regular basis, or has more intimate knowledge of these types of thermal transfer systems could likely give you a more succinct answer and some idea of what to expect.