D600 RAW is not much better than A900
Impressive shot at ISO 25600.
I am sure that the K30 will prove a fine camera for you. And I respect and understand your need for ISO 6400, since I take a lot of shots in nightclub conditions with my A700, and cannot get acceptable shots with ISO 6400, which I would find very useful. (I do very well at ISO 4000, and pretty well at ISO 5000. Some places, however, I need more.)
Keep in mind, however, that there are many photographers who, even were they able to do so, would never think of taking ashot at ISO 400 or 800 or above--many professional photographers among them. Please don't make little of their needs.
DXO does the most professional sensor testing that I know of. In their tests, the K5 (which uses the same sensor as the K30) does very well, overall, within a point or so (which in real life is not noticeable) of the D 7000, the D5100, the A580, and the NEX-7. However, although among the best apsc sensors ever tested by DXO, the K5 does not match the D600, which tests a couples of magnitudes better than the K5, including what DXO calls their "Sports", or low-light noise, measurement: the highest ISO at which a camera can be set and still retain high image quality. The D600 ranks second among all camera sensors ever, second to the D3s, and ahead of both the D3 and the D4, as well as both D800 cameras, achieving a rating of 2980. By comparison, the K5 has a rating of just 1162, many magnitudes worse.
Of course, if you look at the detail of the the DXO tests, at ISO 6400, the K5, while still behind the D600, does relatively much better, so much so that, if ISO 6400 and above is pretty much all you're taking, and you don't care about other advantages, maybe the difference between $800 and $2100 really isn't worth it. But if you're like most of us, and most of your photography is at ISO 3200 and below, then the difference is huge.
If we simplify things a bit (or a lot...), imagine the electronics being a fixed size (as small as reasonably possible), thus blocking a fixed sized portion of the light reaching a photodiode. The smaller the photodiode, the larger the relative portion of light being blocked, the more loss of photons per fixed unit area.
Again, greatly simplified, but that's largely the reason why small pixel backlit sensors gain back much more light per fixed unit area than large pixel sensors. For 1/2.3" 12 MP sensors that can mean up to a stop diference in light or noise. Keeping those tiny electronics constant, that's much less than 1/6th of a stop for current FF sensors. Which doesn't warrant the extra costs at this point. By the time we see well over 100MP FF sensors, you'll probably see this technique making its way into such large sensors as well, because the pixels will be small enough to benefit visibly from it.
Clever explanation because I could understand it
I would say that you are dpoing pretty darned well with your English, Michel.
Thank you very much for your kind incentives, Dulaney