That's what I did. It works pretty well, but I don't have a bunch of water blocks lying around to compare it to. In addition to surface area and turbulence, it also increases the diameter of the water path.
Two things to watch out for. First, I had a bunch of small metal scraps stuck in the threads, and I didn't want them to jam my pump or anything. I sent bursts of pressurized air through, but I still saw some small scraps hanging onto the side of the threads. I kept going in and out with the threading tool, with a good amount of oil on it, and it seems to have worked. Second, getting all the air bubbles out is a 8!+(#. The damn surface tension makes them cling to the threads. Make sure you get them out, because air is a poor conductor of heat (which is why you're water cooling, right?

). I kept tapping the block with the pump running water through. I think if you use water wetter or something else that reduces the surface tension, it should help.