I would think that the D600 is already "fixed". It just seems that the original mechanism suffered from poor tuning. Same seemed to be the case on early D7000's, though not to this extent. It's a big black eye, but I think that the clamour will fade away just as it did with the D7000.
Nikon released the D600 4 days ahead of the Canon offering...they pushed it through. Most folks say that the 6D is better made than the D600 has a faster AF and no famous Nikon "green tinge"...and no crap on the sensor!
On the other hand KR claims that the D600 is superior to the D800... LOL
I guess you would just have to hold both of them in your hands and see which one you think is more robust...
Just read a new post on another forum that I frequent about a person who sent his D600 to Nikon Canada because of the monthly "spotting" issue...lol...they replaced his shutter box! His camera was one of the first in Vancouver btw
If I owned a D600 I would be sending it in to get a new shutter box if I were you all. You spent a running ton of money and if they have fixed the problem you deserve to not have to clean your sensor every few hundred snaps I'm just saying...