Personally I don't have a need or want for that many MP's. I'm also blown away from the performance of the D4, its like driving a race car compared to my D300 and D200. I truly get excited every time I shoot, its an incredible tool.
Also I would expect low light performance to decrease slightly, the D3X was not a low light performer compared to the D3 and D3S. But if you spent most of your time in a studio or have control of you light it may be better for you. But theres no way to know, the D800 seems to have very good low light performance, though not as good as a D4..