I have this question too. I'd like to upgrade from 1Tb to 2TB for less than $150. ;-) If you've seen anything about the SD card slot and whether it will be able to accommodate flush mounting of standard SD cards, that would be helpful too.
So, the speeds of the whole communication of the new mbp,
are in such high speeds, more speed than using a hub with sd slot,
that a user could really consider to heavily use a pernament sd card in mbp?
I personally don't use SD cards at all. I take photos/videos only with my iPhone.
So using the slot as slow (cheap) storage would make sense for my use case.