I've thought about this before. I think the 600º rule is for a stockish truck. Obviously if you added no more fuel, and had a free flowing exhaust, open intake, big intercooler, and say a BB
turbo your EGT's would drop at the same cruising speed (amount of work) but you are still burning the same amount of fuel to do the same amount of work, so mileage would remain the same.
The caveat is the increas in efficiency you have gained by the mods. The IC
, and turbo and intake all contribute to the temperature and therefore density of the air that enters the engine. With the same amount of fuel, and more air you have now increased the efficiency of the engine, and it should be able to do the same amount of work as before on less fuel.
So I guess the answer to your question is "yes and no".
The egt's dropped because the exhaust is not as restrictive, however IF the exhaust makes the engine more efficient, it is now doing the same amount of work as before with less effort.