WAP5 locos have fully suspended traction motors and hence are more suitable for high speed operations. WAP7 has partially suspended traction motors. Moreover, the difference between the HP of a WAP7 and that of a WAP5 isn't much. WAP7 has only 350 HP (560 HP if you consider the continuous HP at the wheels) more which can be neglected in case of trains like Rajdhanis, Shatabdis, Durontos. You will find a significant difference between their accelerations only when they are hauling 24 heavy self-generating coaches. Even then, the difference is only 72 seconds as a WAP7 does 0-110 km/h with 24 coaches in 240.2 sec & WAP5 does the same in 312.1 sec.