Of course those Nix30 will be better than modified OEM cartridges, but IMHO the shock should be upgraded before the cartridges. You can install different springs and oil in the forks as part of a normal service, no additional labor/time, required. Doing any change to the stock shock is a PITA, and say, if you want to try a different oil in the shock you're looking at a $250+ rebuild (350 on the first revalve/respring/rebuild, if you're not quite satisfied, you're looking at another 250... already halfway of what a new aftermaket shock would've been). That's why I rather go with an aftermarket shock right away, on the forks you can easily play with different oil weight, air gap and what not.
That's not to mention that a few years down the road, if you decide to sell the bike, a modified OEM shock is worthless. A good aftermarket shock will hold 1/2 or 1/3 of its value.