I hate to ask, as it has probably been beaten to death all over the world, but I need to know. My 5dII does fairly well, and based on my needs seldom pushes for TOO much of an increase. However, if they are using a sensor very close in MP size, it seems they are just following the approach by Nikon in the D3s revision.
I would expect images at 25,600 to rival 6400 on current 5d. Two stops is asking a lot, but I really think after 4 years of technology development this would be possible--even at 22MP.
If there is merely a clear one-stop advantage, I don't see a worthwhile reason to upgrade if AF is unimportant. Maybe low ISO/Shadow noise improvements could warrant a reason. As they always say, the proof is in the pudding. The more I look at the D800, which on paper appears to be wonderful at its price, the more I believe it will be noisier than a bunch of bad kids on Saturday morning. And that is a major reason NOT to jump ship!! At least for me, it is.
I think your technical understanding of photography is off the mark, to say the least.
Noise improvements happen from two areas, noise reduction and sensor technology. Seeing as you can just apply the latest noise reduction to any raw file the advantage doesn't go to any camera in this regard.
To give you the simple answer no the camera will not deliver 2 more stops of noise.
In fact in 9 years since the earliest days of DSLR's Canon has only increased sensor sensitivity by a QUARTER stop. To give you an idea of how off the charts what you're asking is, not only would it be 16 times the improvement Canon is used to delivering, it is actually against the laws of physics to improve the current 5D II 2 stops at the sensor level. The current 5D gathers 33% of the light that hits it. 2 stops would mean that the camera gathers more photons than actually exist. The camera would have to create more energy than it takes in which is along the realm of perpetual motion machines. Perhaps in the 5D Mark 1,003 but not the 5D mark 3.
It's worth mentioning though that Canon has let their sensors drift behind the field in terms of noise. So the 5D III is poised to have a massive 0.25 stop to 1 stop increase in noise handling, most likely a 0.33 stops or 0.5 stops to catch up to current standards. This sort of leap is considered the equivalent of going from air travel to space travel so it's a big deal and will be the biggest improvement that Canon has ever done in noise from one generation to another largely thanks to pressure from Nikon which has made extreme leaps in noise handling. Canon has already caught up the rest of their line to Nikon which has been done in smaller jumps thanks to a shorter life cycle so a leap of this magnitude is very likely.
So yes while the 5D II > 5D III noise won't meet your standards for being worth it, it will likely be the biggest improvement in a single generation that ever has or will happen.
As far as JPG's go you can expect 2 stops though as Canon implements revolutionary new noise reduction algorithms which have been available in only photoshop plug-ins so far.
