Good to know... but there has to be a depth the circulator becomes somewhat ineffective. I used a coffee maker pump on my home built sous vide rig. The tubing was set up so the cooler water that settled to the bottom of the vessel was pumped to the surface on the opposite end of the vessel. There were no significant temperature gradients (at least when checked with an instant read thermometer.
Does the circulator simply move water straight across or is there up/down circulation? This would be easy to simulate with FLUENT or a similar thermodynamics simulation package.