I had the Flex-a-lite Xtreme 180 on my '06 hemi,.. for the diesel's I think they say to not tow with them because of overheating the transmission,.. but don't quote me on it.
With the HP gain over a mechanical(almost 10hp with the hemi) I was almost to the point of overheating pulling long grades and the fan couldn't keep up.
I am sure there are some good fans out there, just research before you install,.. I would definitely install an override switch as my temp control relay assembly would flake out time to time and would not cool my engine.
As far as i know mechanical diesel fans for today diesel (2000 and newer) pull 6000+ cfm where the biggest flex a lit will do is 5500 or less. I think some of the newer dodges pull upwards of 7000. To tell you the truth on a diesel i dont think you will see any noticible gains. 10hp in a vechicle will not alter your performance nor you gas milage. If it was a race truck or street/strip well yea do it up.
I know we did up a 98 chev 6.5l (not the shinning star of diesels) and had cooling issues pulling minor loads around 5000lbs.
Not after the horsepower, looking to acheive faster warm up in winter on a 04 lly duramax but would hate to exchange the electric fans for the stock fan when it comes to towing.