I upgraded my nominal 400GB SanDisk micoSD card to the nominal 1TB card in the slot on the back of my Surface Pro 3. The computer recognized the card, and reported 953GB of usable space. The card comes formatted at exFAT, so the card should be reformatted as NTFS to take advantage of advanced file system features in Windows.
Windows is showing between 50 and 60 MB/s transfer rates when copying large amounts of data and files of various sizes from another microSD on the builtin USB port on the Surface Pro (with USB-A to microSD USB3 adapter).
One rather large downside to using the microSD card is that deleted files on the card do not go to the recycle bin, and you will not have the un-delete option in explorer by default (Windows does not see the card as a "drive").
I have set up my Onedrive directory at the top level of the card, and the it is syncing without any issues.