From what I can find and post here, the few hundred RPM difference within the 2,000 to 2,500 RPM range relative to E350 and F450 models impact Torque by about 20 to 25 lbs. feet. (Values are RPM/TQ. 1000/358, 1500/375, 2000/380, 2500/400, 2750/410, 3000/400, 3500/395, 3500/390, 4000/375, 4500/300).
Horsepower, a curve indicates at 2,000 RPM, HP is approx. 150, where at 2,500 RPM it's approx. 175.
However, the Torque and Horsepower curves cross at right about 2,200 RPM (170 HP & 400 lbs. feet Torque), where the E450 turns at 65 MPH. Perhaps that particular occurrence contributes to the noticeable difference.
You'll have to compare the values to what your engine turns at another speed.