Brian... I upgraded from a D7000 to a D600 this past December. I still have and shoot the D7000.
To your questions:
1) I am having more of a dust issue with the D600 than I did or do with the D7000; I clean the D600's sensor with a blower and Arctic Butterfly often. My dust problem is related to changing lenses not with a design flaw or a manufacturing mistake. I do understand that the D600 sensor is much larger with more resolution than the D7000's. So sensor dust is really my only issue with the D600.
2) Yes, most definitely the ISO performance is much better than the D7000's. I rarely went above ISO 800 on the D7000 where ISO 3200 on the D600 is very acceptable.
3) I have an iMac 27" with 16GB of RAM and have not noticed any slow-down with Photo Mechanic or Aperture.
4) For my general use (I don't do birding much), I haven't really missed DX's "reach". The D600's files have more resolution so cropping does provide acceptable photos. At recent air show, the D7000's photos crop to 700 KB file while the D600 cropped to a +1 MB file which resulted in much nicer photo. As a side benefit of FX, I don't have to do the 1.5 FOV factoring so 24 mm is 24 mm; it's like shooting a film camera once again.
With regard to migrating from D7000 to a D600 or DX to FX, I'm glad that I did. Even if a D7200 was in consideration, I would have still made the leap to FX but my lens set-up was directed towards an eventual move to FX.