No, not back heat, side heat. Stack the drawers and run the flexwatt up the sides near the rear of the tubs, not the back. There are quite a few larger rack systems that have this and it's supposed to be pretty efficient. I would think more so than with the back heat.
If you can heat the room to about 82-84, then have the two strips of 11" flexwatt going up the sides of the "rack", that would work out okay.