The wise thing to do is to up spec the shifter to X01 and down spec the rear derailleur to GX not the opposite way. The shifter dictates the feel of shifting, the accuracy and the futures like multi up or down shift. The higher group RD just becomes a more expensive target for rocks and branches. This is just a marketing way to spec a bike because from far you can see the big X01 logo on the RD while the rest of the drive train parts are one group bellow (GX, descendant)...