I also just had an idea to increase the efficiency of the heat transfer of copper tubing. I've seen 'turbulators' sold to increase heat transfer in piping. Its essentially a bent piece of metal that is inserted into the pipe that mixes the fluid as it goes through. This reduces the boundry layer and eliminates laminar flow. It also increases pressure loss, but as long as it is accounted for, you should be fine. Here is an illustration.
Now, I don't want to have to buy or even these turbulators. However, I like the effect they have on heat transfer. What if you were to simply crimp the tubing here and there every few inches or so just to stop laminar flow? I would think you could get roughly the same effect.