I mentioned for the reach of the DX mode as I believe you prefer to use your D700, which doesn't have it??? I run a couple of tests using it yesterday, un-scientific as it was only the fence, but looking at the pics I can't see no loss of iQ if any. How it would show up on your Pro glass only you would know.
Maybe worth having a trial Bill to see if its for you, take some of your glass to your local shop with a memory card.
I have a D7100 Simon, it has a x 1.3 crop on the 24MB sensor ..... I thought that it would be really good for my "bird shots" which are crops 90% of the time as it is difficult to get near ........ when I first got the D7100 I used this crop quite a lot ........ I just found that I was better off using the full DX sensor and cropping in LR which is easy and is one of the first thing that you do with all "bird" shots ......... I then took this forward to the FX D700 ...... and I found that I take "more comfortable" shots with the D700 and cropping.
Nikon have got to a stage that it appears that they will not replace the DX D300 ...... Nikon are really saying if you want to shoot DX at a consumer pro level, (and in order to get the other functions that you find very useful, it's a D610 upwards), buy a FX body and use the inbuilt crop function.
DX lens, (lenses) especially at the wide end can be a disaster on FX bodies ....... sensor technology is getting cheaper and if you are buying a DSLR (physical) sized body what's the point in having a smaller DX sensor ....... if you want a small sensor buy one of the many non DSLR sized bodies, the DSLR is an old fashion, maybe "out of date" design .... an SLR brought into the digital age ......... I use the 10MB Nikon V1 to great effect and Nikon have had the sense to design the FT-1, (which they now bundle in the US with the V3), to persuade Nikon users, with all the glass that they have invested in, to continue with them ......... what we have to look forward to is a Nikon mirror-less body, (the V1 is), with an FX sensor and a crop facility were you can also use the very good CX lens, (using a x 2.7 crop - which is far more useful, for birds, than the x 1.3 stuff), and an improved FT-1 that will allow use of a multi cell AF system and all the other function of the camera with Nikon pro glass........... if Nikon do not do this they could well loose out to others, i.e. Sony AR systems, especially for new users ....... when using a lens over say 200mm you are always fighting "movement/vibration" to get sharp images ............. just IMHO of course .... and I always speak from a bird shooters standpoint .. which is a minority view
The swivel screen on the D750 would be useful