I have been debating buying the 6D. I like the feature set and am interested in having a full frame camera in the collection. Today I am using MFT and the Canon APS-C 7D. the 7D is a remarkably capable camera. I think Canon my have overshot on some of the performance qualities on this camera like frame rate, and buffer size. It falls down on high ISO noise. It's performance here is definitely no longer competitive with current offerings. It's a shame because otherwise the camera is fabulous.
So why the talk of the 30D?
The 6D and the 30D are very close in pixel pitch.
Canon 6D = 6.54 um
Canon 30D = 6.4 um
Canon 7D = 4.3 um
(One thing I didn't yet say is I use my large DSLR, among other things, for wildlife and bird photography. THIS is what has kept me on APS-C for so long.)
So back to this comparison. In a perfect world where we ignore sensor noise, these pixel pitches would indicate that the same lens (say a 100-400) fitted to each of these cameras at the same focal length will result in the 7D getting the most detail followed by the 30D and the 6D being almost the same. Put another way, viewed at 100% in photoshop, a subject (a bird for example) will be largest on the 7D and nearly the same size on the 30D and 6D.
Now with that excursion into hypotheticals done I can cut to the meat of the question. When I upgraded from a 30D to a 50D to a 7D, I never felt like I got some revolutionary increase in resolution. The increase in resolution (decrease in pixel pitch) never really increased my image quality by the amount you would guess from the increase in megapixels. I would describe it this way: I increased megapixels, but the new megapixels weren't as good as the old ones (because of noise) so I didn't really get in increase in print sizes for my bird photographs. I was improving resolution a little bit, but not linearly.
The comparison I want to see, and I can't find anyone doing this, is to set a 7D at ISO 800 and a 6D also at ISO 800 and using the same lens and the same subject to evaluate the amount of detail. At ISO 100, the 7D would probably win, but wildlife photography is rarely done under ideal lighting, so I picked ISO 800. I could just as easily pick 1600 or 3200 where the 6D might easily run away with the win.
How much better are the 6D pixels than the 7D's? Does the improvement in per-pixel quality make up for the decrease in resolution between the two and diminish the APS-C advantage for telephoto subjects?