Non-Equilibrium Molecular Dynamics simulations have been used to investigate the kinetics of the bcc to hcp phase transition in ramp-compressed [001]-oriented single crystal iron in the presence of defects consisting in micro-voids. Depending on the ramp maximum pressure, the microstructure of the daughter hcp phase is observed to be strongly affected by the plastic activity in the parent bcc phase. The hcp phase is first nucleated around the pre-existing microvoids, then it grows along preferential crystal directions and the subsequent volume reduction induces a local relaxation of the pressure. The velocity of the bcc-hcp phase boundary is shown to increase gradually with maximum pressure up to supersonic values. For high over-compression beyond the transition onset pressure, the phase transformation becomes almost instantaneous and essentially independent of the initial defects. Meanwhile, the ramp wave steepens with propagation distance and evolves into a shock front, up to the over-driven regime classically observed in both experiments and hydrodynamic, macro-scale simulations.