Andy,
I tested an 810 and IQ is no better than the 800E. I found no IQ increase at all when using the 810's electronic front curtain shutter (which is supposed to eliminate/reduce vibration). The monitor is slightly better for menus but the position of the bracketing button is a complete disaster - they should have left it where it was. If you currently use Capture NX2 you won't be able to with a D810 because it won't read the NEFs. Capture NX-D (the supposed replacement) will read them but it's a horrible piece of software.
That's my 2p worth.