Tamron 18-270mm for use with D800
Not correct, tamron offers a 28-300 VC that is full frame. BTW much cheaper than the Nikon. I had this lense and used extensively on a D700, reasonble between 28-200 but got soft very quickly after that, focus was also not so fast or decisive in lowlight.
From the reviews of the 28-300 Nikon I think it probably justifies 80% of its premium, and on a D800 you probably want it, or a FX user should just settle for a D700
The only super-zoom type lens is the Nikon 28-300 3.5-5.6G.
If you need this kind of range, this will get you close, with a bit less on the long end.