Yes, cooling to very low temps is an issue, that is why what I am looking at is advancing the ability to reach equilibrium in temps. One issue is the creation of turbulence within the heatsink while minimizing resistance. You see, ideally you want turbulence in a heatsink so that the molecules will not get stuck as a result of fluid dynamics.

Heatsinks that have the highest turbulence and contact area get the best performance assuming that the pump is powerful enough to move the water in it.