"Recovery" is a different topic and I'll defer to Marianne on that for now. I'm more interested in fundamental dynamic range while "recovery" implies "pushing the envelope".
FWIW, for the D700 ISO 800 FX is not equivalent to ISO 400 DX, more like ISO 500. You would read this as the horizontal difference between the two PDR curves. Note for example that for FX ISO 400 and less there is no DX equivalent ISO (at the same image quality). It makes sense that you can't get FX low ISO performance out of DX Crop Mode.