Will a new D800/600/D4 improve my work?

Like others have said it depends on your needs. Personally for me I do feel it has improved my photos. It doesn't help with things like composition or understanding light, but it has some advantages that matched well with my goals.

I shoot mostly landscape and cityscape and the D800 has great DR. I can get shots now that took a multiple exposures in the past. I also think it handles WB better and it opens some doors for me with wildlife since it has amazing crop ability.

I have always been drawn to scenes with large DR so I now can push things even further with the D800.