I think Sharpness is like money. (we always want more)
On a serious note, hv u considered to perform AF fine tune for this lens on ur D600? (LiveView Vs ViewFinder).
it's a very quick way to determine if you need fine tune or not. This will solve if u have any focus issue. (I do that for all my lenses on my cam).
Good luck.
if u worry about centre vs corner sharpness, as 1 of the bros here pointed out, pls check photozone.de for MTF (resolution) for this lens.
u will see that corners are never as sharp as centre (as all lenses in the world).
But 24/70 is very very sharp in the centre. So keep shooting..!