I think MSC only applies to what is 'normal' without taking status into account. Which means that normally, your ticket would only be allowed for say J allowance, then its whatever CX allows that will be honoured throughout.

So if its say just economy, its 2x23kg for CX flights, and that will be printed on your ticket as 02P. AA will then have to honor that allowance as MSC. AA does not have to honour what CX would give you on top of your normal allowance. But they will give you extra allowance as OW Sapphire on you AA flights.