Here's the bottom line: horsepower = rpm x torque / 5252. Essentially, if you have low torque, you'll need to increase your RPMs to get higher horsepower. Think of a Formula 1 car with low torque revving up to 15,000 RPMs. Some have gone to 20,000 RPMs.
I think Diesel folks like having more torque because the engine doesn't have to turn as fast to produce the same horsepower. Less RPMs equals less heat, noise, and friction. But the easiest way to increase horsepower is to increase RPMs.
But the only time you are near your maximum horsepower is when you are close to redline. Usually during acceleration or climbing up a mountain.