15kts for 10T seems quite a lot. True, from performance criteria you could use V1 & V2 for the higher weight. But for Vr, if you try to use a speed 15kts faster than required you might find yourself 'holding' the a/c on the ground as it wants to loose its earthly bonds. Perhaps not a good idea. V1 can then = VR for the actual weight.
What would you do on an empty ferry flight?? By definition a ferry flight is empty, IMHO.