I do not have this lens but to have a flat field, you need an orthoscopic lens. As far as I know, the Macro lenses are orthoscopic (more or less) and I don't know any zoom on this very planet who is orthoscopic, even the "macro" zoom.
So, if you really want a good 50 go prime and if you really need AF go for the 50mmf2.8 Macro SIGMA (who resolve very well the D800). Otherwise, jump on a MACRO 50f2 Zeiss.
I might be wrong, but I'm curious and wait for ppl giving me samples out of the new Canon (but I doubt).
If you don't mind going a bit longer, the 70mm Macro f2.8 SIGMA is on of the most orthoscopic lens that exist (in regular Market).