You would need to ensure that the reduction in power is equal to the reduction in speed^3.
So 5.5kW x (2875/2900)^3 = 5.36kW - So to operate at 2875 RPM, your motor must be be able to produce 5.36kW at that speed otherwise there will be too much strain on the motor.
THe pump is unlikely to be damaged by this though.