I have said this many times. You CAN NOT compare dynamic range on ISO100, and assume everybody works in ISO100, and that this is the correct ISO to measure by, like DxO does. I RARELY go below ISO 1600 for my kind of work.
You do realize that at base ISO, the D800 has more than 2.5EV more DR
than the 5DIII, and at worse, i.e. at ISO 25,600, it has 0.26EV worse DR
than the 5DIII. So, compare at whatever ISO you'd like. The D800 is rarely going to do worse than the 5DIII, but can
do much better.
Furthermore, the future is likely 'ISO-less'. In other words, ISO becomes metadata written into the file, much like white balance. Amplification is done during RAW processing. That way, you actually retain the DR of base ISO even at high ISO (w/ low enough read noises).
We just need manufacturers (both hardware & software) to implement this. Personally, I would've already implemented this w/ the D800, given the low read noise of ~3 e-.
But maybe the industry is waiting for a read noise of 1 e-, or a 16-bit ADC.