First, what is in D800 that is particularly good for wedding and events?
My experience with events (wedding should be similar) is "shutter speed". And 21MP or 36MP are both more than enough in pixel counts.
To achieve high shutter speed, low noise at high ISO is important.
As I mentioned a few days ago, 5D3 plus f/4 lenses should be good enough, and should beat (at least match) D800 plus f/2.8 lens in terms of image quality (you can use your primes for bokeh). This package is also lighter (U96 can hold a camera + 70-200 f/2.8 for 30 min. I am not sure how much stronger you are in holding a camera + 70-200 f/2.8, but with f/4 lens, you should do better).
