OP wrote above:
"Got an email from Transcend saying that they still have no time frame for even beginning development of an upgrade option."
I would not buy a retina MacBook Pro with an intention to "upgrade the SSD" in the future -- because in all likelihood, the chances of such an upgrade becoming available are quite small at best, and zero at worst.
Buy the MacBook with the size of the drive you need for the life of the machine...