There's no plateau for DEX, the more of it you have the better as it is NIN's primary stat. Potentially, in certain cases (maybe some of the Arachne), the primary+secondary totals on a particular piece (say, an over-melded craft item) can out-weigh a higher iLvl piece, but there isn't a given DEX cap where you can start stacking in favor of the secondaries and completely ignore more DEX.