Cummins power

HP = Torque x RPM so more RPMs.

There isn't really much (if any) fuel mileage penalty because the HP is derived in part from injector matching technology, more boost and fuel pressure and newer variable geometry turbos. So the power comes in great deal form increased efficiency.
Any cost in FE is negligible.

The new engines are all computer controlled. They can dial in a range of horsepower settings and torque. Often times turning these engines down reduces their fuel economy.

When my company turned our truck engines down and limited them to 63 mph they lost fuel mileage. When they complained to Detroit Diesel they were told it was their fault for turning the engines down. A castrated engine has to work harder and thus the mpg suffers.

That may explain part of your question.

I think the 300 HP is the 5.9L (older version). The others are the 6.7L