There is a formula to calculate how far a vehicle travels at X speed and it is farther than you would think so those numbers seem in line.

Having just put slightly better rotors on, I do not expect substantially better than stock as the cuts only help to dissipate heat during prolonged braking but should have little effect on regular use. braided lines, better shoes, dual piston calipers, and or/bigger rotors would be the only way to substantially improve overall braking on this heavy vehicle.