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Flexible Data Placement (FDP) promises to reduce write amplification by steering writes across reclaim unit handles (RUHs), yet outcomes vary widely across devices. This paper presents WARP, the first open emulator and comprehensive study of FDP SSDs. Our cross-device, cross-workload characterization shows that FDP sustains near-1 WAF when RUH isolation aligns with object lifetimes, but fails under misclassification, RUH interference, or adversarial invalidations. WARP reproduces hardware WAF trends while exposing per-RUH dynamics and configurable policies hidden in real devices. With WARP, we explore the firmware design space for FDP and demonstrate policies that reduce WAF beyond current hardware. By combining empirical characterization with a transparent emulator, this work advances FDP research from anecdotal reports to principled understanding and provides a platform for future FDP-aware system design.