Some folks are not satisfied with the Cummins ISX, 600 hp! But if you don't know how to handle it, the horsepower and torque can get you into all kinds of trouble, and even cost you lots of money.
I am very happy with my mechanical Cummins 6bt 5.9 (230 hp), but then a Cummins mechanical 6C 8.3 (300 hp) sounds real nice. Both are very easy on fuel and much easier to work on than todays computerized engines.
And I haven't had any turtles pass me on any hills lately. But as someone who has pulled over 130,000 pounds of steel coils on two trailers, with a Detroit 318, and 80,000 pounds (8700 gallons of gasoline) with a Detroit 238, I'm quite comfortable with folks passing me. Just smile and wave as they go by. I usually got to pass them further on down the road.