Why do you think that shooting an FX body in DX mode is silly? Although the D800 is not a "sports camera" (as you point out) it seems to me that it might serve the purpose in DX mode because it (i) provides the crop factor for composition; and (ii) in that situation, 15MP may produce more images that appear reasonably in focus than 36MP.
If the D400 is produced in the profile most speculate about (DX mode, 24MP, pretty good fps) I will want one. In the meantime I am considering the D800 to serve the action camera purpose.
Do you think that the D600 is a better action/low light camera than the D800? If so, what is the advantage?