Same problem here. I understand that the page that details the specifications of each laptop model, refers to the specifications that bring the different configurations that Asus has designed for that model, but does not say, in anywhere, that allows, or not, up to certain storage capacity, or other, when upgrading.
I also understand that Asus does not want to commit claiming that these models can support, for example, a SATA3 M.2 of 1T, without having tested it before, what I can not understand is that, today, Asus has not yet done those tests and their customers have to be waiting like idiots, or worse, betting our money to that our laptop, which costs more than 1000€, will recognize the SSD that we need to bought.
I need to install a Sata3 M.2 500GB SSD in a GL753VD ... can I? who knows.
Excuse my low level of English.