Congratulations on the D600. The camera shines in low light. I just took a series in a dimly lit church (no stage lighting) and the exposures (at ISO 6400) were exceptional for lack of noise and just plain looking so normal. The other consideration is that, shooting in a limited space, it is much easier to fit things into the image with FX. I suspect you will enjoy the experience.
I'm another statistic in the move to LR5 - I know there will be at least one weekend lost to moving legacy software and files around so I can update my OS to accommodate the new LR. Grrmph.