Speaking ex cathedra is restricted to matters of faith and moral (catholic doctrine) as part of the "sacred magisterium" power.

Not sure where you got this notion that the Pope can speak ex cathedra about Church or Curia administration, but he can't.

In theory the Pope has supreme power and could impose administrative reforms by decree, but in practice he needs the support of most the Cardinals in permanent residence in Rome and especially those heading the Congregations and the SoS for any of his reforms to have much success - they're the ones directing the Curia and who would be implementing the reforms.

Whatever reforms of the Curia he's of a mind to do, it's far more likely to arise from consistories and special commissions and take many years, and an early sign of administrative reforms to come would be a big reshuffling among the heads of Congregations, putting in place the right Cardinals. But there are big opponents to ideas of deep reforms and decentralization.