The speed difference is very minor and you will not notice a very strong difference. I checked the processor's benchmarks and there they are:
2.6 GHz: 12657
2.3 GHz: 11685
i5 Mac: 5718
If these numbers don't mean anything to you, then I recommend not spending money on the extra .3 GHz because there is no point in that. Getting a Core i7 instead of i5 is a very good choice because the performance is double than the i5. Hope this helps.
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.
The speed difference is very minor and you will not notice a very strong difference. I checked the processor's benchmarks and there they are:
2.6 GHz: 12657
2.3 GHz: 11685
i5 Mac: 5718
If these numbers don't mean anything to you, then I recommend not spending money on the extra .3 GHz because there is no point in that. Getting a Core i7 instead of i5 is a very good choice because the performance is double than the i5. Hope this helps.