With large vehicles, whether it be our buses or over the road trucks, need large increases in horsepower and torque to make any kind of difference. For instance, my bus had 300hp and 800lb/ft torque (brown tag N65 injectors). After turbocharging and air to air intercooling with 9G75 injectors, the engine is putting out 375hp and 1125lb/ft torque (on an engine dyno). About an 29% increase in torque. The difference is, when pulling my car going from L.A. to Las Vegas, without the turbo, there were several hills that I would have to down shift slowing down into the lower 40's mph. Now with the turbo, the only hills I have to down shift on is the Cajon Pass and Baker hill. Pulling the southbound I-15 hill out of Nevada in California, my speed has increased from 32-45mph. Doesn't sound like much, but you do get up the hill much faster. It just makes driving more enjoyable. Plus at altitude, no more smoke.
In trucks, the same thing. At 80,000lbs, the normal torque that most trucks have now is about 1550lb/ft. Increasing to 2050lb/ft (which is an expensive option [bigger radiator, bigger transmission with cooler, bigger rear ends and U-joints) increases the hill climbing probably not as much as you'd think. A 1550lb/ft torque truck will pull the south bound Grapevine at around 35mph (much better then the 28mph I used to do with my 8V-92TA). Increase to the 2050lb/ft engine, and you'll do the same hill at around 50mph.
So you's makes you's choices. More power you have, more expensive it is to run (fuel, maintenance on a big engine is always more in proportion). Good Luck, TomC