I think you have more performance boost on D800E raw files by using NX2 than by adding 0.3 Ghz.
If you use a D800E what it is good at: competing with system/large format camera's on architecture, product, fashion shots, invest in NX2. It will pull much more out of the raw files than Lightroom can do. If you do things a D800E is not good at: like news or weddings: trade it for the new D810. It will have sRAW, and allows you to shoot on 16megapixels on the high-speed crap shots. That will decrease processing time by a 55% instead of the meagre 10% the 2.6 offers.