I agree with your analysis. If one cares about wide angle photography one should prefer the D800. It is an awesome camera. However, as you say, if one wants to shoot only macro, a DX format is better. The D800 is not much better in pixel density than the D300. I am avoiding any more DX lenses but the DX bodies still interest me. Unfortunately pricing a Naudicam D7100 housing that is the rough equivilent of my metal SeaTool D300 housing comes to $5000. That is a tough nut to upgrade to a D7100.
Maybe moving to FX is a better plan for increased flexibility. I admit to being in a quandry so i am still in D300 mode.