I installed one on my previous coach, 2005 Monaco Diplomat, rear radiator ISL 400. It doesn't increase horsepower, but returns horsepower that was used to turn the OEM fan. On the ISL, it was a 25HP increase/return. You could feel it under acceleration and when passing. On smaller engines, they get better results because they're turning faster. The ISC was said to get 40 HP.
I also looked at the Horton which used an electronic clutch. It was a lot more work and was no advantage when hot out or climbing.
There was no cooling loss with the Source Fan, it was identical to the stock fan. I would buy another if I had a rear radiator coach. I also added the DIGICR chip to my ISL which added 52HP. My 36' Diplomat was a rocket, even on the grades, towing my 5500 pound truck. As far as I could tell, there was .3 -.5 mpg.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab