Ultrafast plane-wave ultrasound imaging involves emitting multiple plane waves steered at different angles to obtain multiple low-resolution large-area views of an imaged medium, which are then coherently compounded to form a high-resolution image. Such coherent plane wave compounding (CPWC) allows for much higher frame rates than focused imaging, thus enabling novel diagnostic ultrasound applications. This work describes an improved frequency-domain approach to reconstructing CPWC ultrasound images of a stratified medium, where the sound speed may vary with propagation depth. Our approach adapts a well-known phase-shift migration technique from geophysics to the CPWC setting, simplifies an existing plane-wave migration model, and yields more accurate imaging results.