In reference to the heater outlet temperature that one can successfully meet the vcm spec. etc. on the contents of the drum. Please be aware that these temperature transmitters can read false. Thermowells do coke as run length’s are increased. Please monitor the temperature of the drum feed @ the switch valve & the unquenched vapor line temperature. This will give additional indicators to allow for the reduction in fuel gas you are speaking of. It may be that you thermo-wells on the outlet of your heater tube is partially coked thus giving a false reading. I would also look at your recycle rate & percentage of ligher material in your feed. Remember “heat of vaporization” is endothermic and increases the duty on the charge heater. Hope this helps.