In a recent similar thread, MEL enlightened me that there is no comparison between cooling of a 6.9 and a 7.3; something to do with the 7.3 lacking many of the coolant passages that a 6.9 has.
I know that loaded REAL HEAVY and in extreme heat conditions, my old 6.9 with a crappy old worn-out fan-clutch, MAX A/C blowing strong and turbo whistling, NEVER EVER offered to even act like it was going to overheat.
All that being said, I absolutely abhor thermal-lock-up fan-clutches, more so due to very poor A/C performance than engine cooling issues; hence, I have two sets of clutches for each truck, a standard un-altered clutch and one that I have pinned solid.
I put the pinned clutches on about mid-MAY and the un-altered ones on about mid-SEPTEMBER; I see no need for a hurricane blowing through the radiator when it is 30* outside.
I can see/feel no difference whatsoever in fuel-mileage or power between the two.
There is a company that makes electric-switched fan-clutches that would be way more desirable than the troublesome thermal-spring type.
REGARDLESS of engine/vehicle make/model, there are more complaints of poor engine cooling and poor A/C performance due to the fan-clutch than any other problem; YET, people just keep putting up with them, afraid they might lose half-a-horsepower or a couple MPGs.
BIG electric fan set-ups are even better that any engine-driven fan.
Many will claim that the engine-driven fan pulls more air than an electric, but I can't see it.
The engine-driven fan barely crawls along in RPM compared to a good big electric; and, the constant speed of the electric is much prefered to the fluctuating speed of an engine-driven fan; PLUS, one can have electric fans controlled by manual ON/OFF switches and kill them completely in cold weather, or have them going full-blast when sitting still with the A/C on.