All this has been entertaining, if more than a little reminiscent of a few other discussions...
*Pulls pin out of grenade*
But to return to the original post, I'd say the 645z looks like 'king of low light' at present, based on what I've gleaned online. I'd still rather have a 1Dx (or wait for the next Canon full frame body) for the system benefit, as Neuro says. But in terms of low noise at high ISO I reckon the new Pentax wins - as you'd expect, with a bigger sensor area.