Gentlemen,
I have not shot the D800 in DX mode and have little interest nor reason to do so. My own question was simply to determine how many pixels was I going to get in the portion of the frame that would be captured by a DX body. There won't be a quality difference capturing DX or FX mode - it's the same sensor, same dynamic range, etc.
I'm not a burst shooter for much, so gaining a frame or two per second while loosing all those pixels makes no sense for my shooting. I'd settle for fewer pixels if it gave me better ISO performance, but that won't be the case.
I aim to compose with camera as much as possible and I always shoot for the maximum number of pixels I can capture - especially with BIF.
My main lens, with TCs, for the DX shooting I've done up to this point, was the 400mm VR, but just getting it paid off, it was time to invest in my next long lens. I had planned on getting the 200mm f/2, but with expanding to FX, I invested in the 600mm, so I can still compose with my camera, not my computer. The D800 is a game changer on a couple of key elements, and I'm really looking forward to using it in Yellowstone, where I'm headed in a day or two (fourth trip there, this year). I have a D4 on order that will cover the need for better ISO performance and those rare times when I do want good burst capability.
Bottom line, Nikon really needs to get the D300 replacement announced and shipped. This is a key camera for many bird and wildlife shooters. The D4 and D800 have really raised the bar, and the D400 needs to do the same, especially in relation to the D7000 - which i think will be easy to accomplish.