Hi,
I have found contradictory statements about that question in the web. Also, some say that images can be saved in a 16 bit format, but the 16 bit palette might not be available through the image editing process itself. I observed that the gradation curve tool indicates numbers from 0 to 255, which supports the theory that you're limited to 8 bit while editing.
Best,
Michel
PS: I'm importing TIFF, and save them as PNG, in case that this matters.