What if Canon simply ditched the 5D3's sensor and put the 6D's sensor in it?
From the reviews I read and the raw samples I looked at myself you're absolutely overestimating the 5d3/6d difference - the 6d might have 1/3 stop less iso noise, but the 5d3 also has improved banding, has more resolution and is sharper (even with a much stronger aa filter!) than the 6d. Except maybe for iso12800 (favors 6d) or low iso studio shots (favors 5d3) the differences won't show.
I agree about the overestimation thing -- I'm thinking of selling my 60D and getting a 7D (mark 1!) because of AFMA. A FF would be great, indeed much better than my 60D or my "new" 7D, but what's keeping my photos from being better is definitely not my camera.
Going FF with Canon would mean losing the built-in flash as a commander (I have only one flash) and becoming restricted to on-camera flash, when what I need is just the opposite: quit being such a lazy ass and get the flash off-camera more often. Sure, I could just buy another flash, but going FF, buying new lenses and accessories is expensive and probably wouldn't make my photos much better, if any. I'm the limiting factor...
And, as jrista pointed out recently, there are softwares with incredible NR capabilities available nowadays. I've found that ISO 1600 and 3200 are really good after adequate NR on the RAW files of my 60D, and for what I shoot it's really enough.