Apparently, I was wrong about how sensors do high ISO. Interesting.

That said, the DPReview tests and DxO people certainly know what they're talking about, and that's that the D800 has a kick-ass sensor that performs very well at high ISO, particularly if you're willing to shrink things back down again later.

On the other hand, the raw files are almost twice the size of other cameras.

At the end of the day, you buy what fits your budget and solves the problem. If you've got a specific thing that you're shooting a whole lot, then you might make specialized decisions about gear that are better optimized for that use case (e.g., purchasing a really bright prime lens of the size that you might otherwise spend most of your time zoomed). If you lack that sort of specific use case, then this whole discussion sounds a bit like premature optimization. It's not necessary.

Lastly, if you just need something to carry around for spontaneous moments, then you should be paying attention to size and weight as much as the quality of the sensor. You may prefer to skip the D-SLR universe entirely and go for one of these world-beating compact things, like the Sony RX100.