Wouldn't you want it the other way around? Right now you are throttling the power that goes to your thermostat, doesn't a thermostat need a set amount of power to function? You want the thermostat functioning on full power, but if it ever melts ON, can only fire so much juice to the heat tape.
Wall---->Thermostat---->Rheostat(throttle)---->Racks
No?
-JonV