>The D600 will give you an image that's about 15mp with a DX lens.
In DX Crop Mode the D600 gives you about 10.2MP, not 15MP.
>But what about IQ? Is it fare to say that the image IQ from a >D600 with a DX lens is Going to be better than the same shot >taken with a DX body?
In general, yes. That's mostly because it's newer technology than the current top-end DX cameras like the D300s and the D7000. If and when either of those models gets replaced, things may change somewhat.
Personally, I don't like the idea of buying an FX camera with the aim of using it only in DX Crop Mode. The viewfinder image will be rather small, for one thing, and you'd be paying over the odds for something you won't be using.