I don't know a whole lot about the newer machines but i see no reason why the fan can't run at full speed. The faster it goes the more cooling it does. I see your point in winter that it doesn't need to go as fast but fuel consumption wouldn't be noticeable at all.
I'm not sure how many hp the fan consumes, it would be highest at full rpm of course, I would not be surprised that that 5 hp is required. The big rigs and most cars or pickups have some sort of clutch or electric fan(s) for this reason.
If there is room for a larger pulley on the angle drive gear box then it would be easy to slow it down a percentage. Or perhaps electric fans could be fitted.
But unlike a car or other fast moving vehicle there is no ram air effect to get some free cooling.
The easiest way to know would be to compare noise and fuel consumption from a K series to a G or older loader.
Personally I suspect that fuel would not be that noticeable though they would be some quieter.
Most heavy equipment that use hyd fan drives do so to allow reversing the fan to blow out dust, leaves etc